This lens showed up in the market in 2003 and generated a shock wave in the photographers' world, especially among Nikon equipment users. A lens that could cover a range of 18-36mm on an APS camera (after the crop factor) was quite rare at the time.
Until then, the only thing that existed at Nikon and other manufacturers in the wide range were full frame lenses with very mediocre optical quality – but at a scandalous price.
When this lens was announced and its price was known, the Nikon users had a blow: its initial price was about $1000 US – a very expensive pleasure.
After the lens finally entered the market, and its initial tests were published – it created, once again, a tumult.
The testers firmly claimed that "it's too good to be true".

For over two years, this lens was the sole player in the field, until other companies – Canon and three third party companies: Sigma, Tamron and Tokina presented lenses with a similar range.
When holding this lens in ones hand, it feels light and yet firmly built.
The lens has a convex front element, as do most wide angle lenses, which usually ensures a fair amount of flare.
I have been using this lens almost since the day it landed here and I am enjoying every minute. I take this lens with me whenever I can.
This is how the lens looks:

Color Space: 65535

The MTF curve looks like this:

 

We can see that this lens provides amazing sharpness with a wide open aperture and its sharpness decreases towards the edges.
The following are its technical specifications:

The specifications reflect the amount of investment in this lens. Three aspheric elements and two structured ED elements, as well as a curved aperture (although it is made with 7 leaves only).
The lens quickly focuses AFS – and shooting from close range (30cm), not exactly a macro with its focal length.
Let us get on now with the test results.
The colors it restores are neutral and precise: 

Model: NIKON D200 Exposure Time: 1/2000sec F Number: 4 Max Aperture Value: 4 Focal Length: 24mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 36 Date Taken: 2007:03:30 08:34:45 Color Space: sRGB

It enables close range photography, although not exactly macro:

Model: NIKON D200 Exposure Time: 1/2000sec F Number: 4 Max Aperture Value: 4 Focal Length: 24mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 36 Date Taken: 2007:03:30 08:33:03 Color Space: sRGB

It has a nice bokeh, as can be seen in the blurry light colored branches in the background, quite good for such a wide angle lens. Usually, manufacturers do not bother to invest in a fine bokeh for lenses which are not long tele or portrait lenses, but in this case, Nikon took the trouble to do it:

Model: NIKON D200 Exposure Time: 1/2000sec F Number: 4 Max Aperture Value: 4 Focal Length: 24mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 36 Date Taken: 2007:03:30 08:33:27 Color Space: sRGB

Amazingly, it copes with a back light (direct sun!):

Model: NIKON D200 Exposure Time: 1/2500sec F Number: 4 Max Aperture Value: 4 Focal Length: 24mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 36 Date Taken: 2007:03:30 08:34:59 Color Space: sRGB

It creates deep blue skies (as do very wide lenses) without the need to use a polarizer (an example with a polarizer will follow):

Model: NIKON D200 Exposure Time: 1/2000sec F Number: 4 Max Aperture Value: 4 Focal Length: 24mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 36 Date Taken: 2007:03:30 08:35:06 Color Space: sRGB

As expected, the lens has a small amount of spatial aberrations (barrel and pincushion) which do not affect regular use. It has slight vignetting which is not very much apparent. It has little CA (color aberrations), and that improves when closing aperture.
When closing aperture in other lenses, these aberrations usually get worse. The following are mainly edge aberrations:

Model: NIKON D200 Exposure Time: 1/1250sec F Number: 4 Max Aperture Value: 4 Focal Length: 12mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 18 Date Taken: 2007:03:30 08:33:53 Color Space: sRGB

The most amazing fact about this lens is that it is simply immune to flare. Even with the sun in the frame, it is difficult to catch flare, and in most cases it doesn't appear at all:

Model: NIKON D200 Exposure Time: 1/8000sec F Number: 4 Max Aperture Value: 4 Focal Length: 12mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 18 Date Taken: 2007:03:30 08:34:10 Color Space: sRGB

This is an interesting fact for two reasons – one, ultra-wide lenses love to create direct flare, as well as indirect flare refracting inside the lens because of the convex front element, but this lens has practically none.
Its microcontrast against the sun is excellent:

Model: NIKON D200 Exposure Time: 1/640sec F Number: 4 Max Aperture Value: 4 Focal Length: 12mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 18 Date Taken: 2007:03:30 08:32:15 Color Space: sRGB

Its sharpness in a wide open lens, in short range, at the wide angle state, is very good:

Model: NIKON D200 Exposure Time: 1/320sec F Number: 4 Max Aperture Value: 4 Focal Length: 12mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 18 Date Taken: 2007:03:30 08:32:38 Color Space: sRGB

Even in its tele state it is very good:

Model: NIKON D200 Exposure Time: 1/320sec F Number: 4 Max Aperture Value: 4 Focal Length: 24mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 36 Date Taken: 2007:03:30 08:32:45 Color Space: sRGB

This phenomenon recurs in the long range – at the wide angle state:

Model: NIKON D200 Exposure Time: 1/1250sec F Number: 4 Max Aperture Value: 4 Focal Length: 12mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 18 Date Taken: 2007:03:30 08:35:25 Color Space: sRGB

And in the tele state:

Model: NIKON D200 Exposure Time: 1/1250sec F Number: 4 Max Aperture Value: 4 Focal Length: 24mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 36 Date Taken: 2007:03:30 08:35:29 Color Space: sRGB

The following are four examples taken while traveling – its real use. The first example is the beach in Rio, what colors, and the width!:

Model: NIKON D200 Exposure Time: 1/1250sec F Number: 6.3 Max Aperture Value: 4 Focal Length: 12mm Exposure Program: Aperture priority Exposure Bias Value: 0 ISO Speed Ratings: 125 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 18 Date Taken: 2006:03:04 14:48:11 Color Space: sRGB

The next picture was taken at the Invalides, in Paris, the blue room. Above, we can see the cupola which is almost above our head:

Model: NIKON D100 Exposure Time: 1/10sec F Number: 4 Max Aperture Value: 4 Focal Length: 12mm Exposure Program: Aperture priority Exposure Bias Value: 0 Metering Mode: Spot White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 18 Date Taken: 2003:10:22 14:43:33 Color Space: sRGB

The third picture was taken in the same building over a banister preventing entrance to the room. One shot shows the entire room:

Model: NIKON D100 Exposure Time: 1/100sec F Number: 4 Max Aperture Value: 4 Focal Length: 12mm Exposure Program: Aperture priority Exposure Bias Value: 0 Metering Mode: Pattern White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 18 Date Taken: 2003:10:22 15:02:42 Color Space: sRGB

The last example is a picture taken in Croatia with a CPL polarizer (thin). Real drama takes place in the sky:

Model: NIKON D100 Exposure Time: 1/640sec F Number: 7.1 Max Aperture Value: 4 Focal Length: 12mm Exposure Program: Aperture priority Exposure Bias Value: 0 Metering Mode: Center Weighted Average White Balance: Auto white balance Flash: Flash did not fire Focal Length In 35mm Film: 18 Date Taken: 2003:08:20 15:45:29 Color Space: sRGB

The following is a table summarizing the test results:

All the parameters which are important to ultra-wide angle lenses have successfully passed the tests.
How does it compare to similar lenses on the market?
In comparison to the Canon EFS 10 – 22, the Nikon lens is sharper at all ranges and it has less vignetting. The Canon lens has less CA.
In comparison to the Tamron 11 – 18, the Nikon lens is sharper in all ranges but it has slightly more vignetting than the Tamron and slightly more CA.
In comparison to the Tokina 12-24, at the wide-range they are quite similar, but as the focal length grows, the quality of the Tokina lens drops.
In addition, the Tokina lens has a serious flare, heavy vignetting and apparent CA.

The only lens that compares to the Nikon lens, is the Sigma 10-20. Although it does not have a fixed aperture, its sharpness is similar to that of the Nikon. The Sigma lens has slight flare and slightly more vignetting than the Nikon lens.

The bottom line – is the Nikon 12-24 lens worth the $900US it costs?
If you have the money, the answer is yes. It is fun, sharp, "flareless", and creates very slight aberrations. It is fast to focus and has the traditional Nikon structure.

If you cannot afford to spend such an amount of money, you can buy the Sigma lens which costs far less and will still give you good results, and it has an extra width of 2mm, which is a lot for this type of lens.
You just have to take care not to excessively close aperture with the Sigma since its CA rises when closing aperture!
The two other competitors are not as good as the Sigma and the differences in price do not justify any compromise.
Is the 12-24 practical also for 17-55 users? I think it is. The distance between 12 and 17 mm is tremendous.
Is this the only alternative for ultra wide angle lenses? No. If we really want ultra-ultra, we can buy the 10.5mm fisheye and straighten it with a $30 software program. The results are impressive, as I wrote elsewhere.
And, a surprise – the lens can work with an FF camera – although it is designed for APS cameras of a focal length of 16mm and on.
In addition, if we mount a teleconverter, such as the Kenko PRO 300 x1.4 and turn it practically into a 16.8-33.6mm, it will function on an FF camera throughout the range!
For nature and scenery lovers, an ultra wide lens is a must. The 12-24 is an expensive Prima Donna; Nikon knows its product is good so if you want it, you have to pay!
And, as I said, if you can afford it, go ahead.
A final word of caution: all ultra wide lenses have a natural and normal tendency to distort perspective. When holding it improperly, buildings recline and create unnatural angles.
It is strongly recommended to hold the camera in parallel to the floor, even if you have to climb a ladder or lay on the floor to keep it parallel, unless you want to purposely create this distortion.
In addition, it is also not recommended to take close up portraits or groups of people on the wide side because the people standing at the edge of the frame add several kilograms to their weight and they would not appreciate it or you!