Who this ultrawide (UW) is for:
Do not buy this or any other UW if you're unsure what you're getting.
If you're a happy amateur who wants holiday snaps, pictures of friends, wildlife, sports, portraits, etc. this is NOT for you.
It is primarily for making dramatic perspective-distorting pictures of landscapes, architecture, environments, urban landscapes.
If you're unsure, go walkabout with your existing kit lens (probably 17-55 or so) and never zoom beyond 20mm. Get close to objects, look at long profiles and lines which extend from you to the horizon. You'll also want to look through your picture history and see how many of them were shot at the wide end.
If you want more width, it's amazing what you can get into the picture with this lens and the creative possibilities begin to make a lot of sense.
If this is what you're after, I strongly recommend the Nikon 10-24 over the alternative UW's on the market. Keep reading...
The lens - good points
It's really, really sharp. I'm very impressed.
Excellent colour reproduction - better than any other zoom lens I've ever used - though not better than a (good) prime lens. Very natural but dramatic lighting. Evening skies, light reflections, etc.
Fast AF, works on all modern DX format Nikon DSLRs.
No CA I've ever noticed, flare is controlled (though you do need to look out for flare with any UW, and it does happen).
Distortion seems natural (it is normal for such a wide lens but doesn't jump out with the Nikon).
Nice 10-24 range - UW through to normal/wide
It's "only" f3.5 - 4.5, but this doesn't matter too much - you don't usually need low light capability a great deal on an UW. Night shots generally need a tripod anyway.
Auto-exposure is usually perfect even in challenging circumstances with zero adjustment. My D7000 usually requires -1/3 to -2/3 exposure adjustment with most lenses (a well known 'feature' of some D7000's), but not with this lens.
Focuses down to about 20cm - useful for an UW, coz you're going to get VERY close to some things.
Takes standard 77mm filters, lens caps. Manual/automatic clutch for manual focus override. Nice firm zoom, feels substantial, metal mount.
The lens - not so good points
Slightly heavy, slightly big for the camera. The body will lean forward round your neck when unsupported, with lens pointing down.
Reasonable non-pro build quality. Good, but not great.
Expensive, but look around for the best price.
'Only' goes to 10mm vs the Sigma 8-16.
My decision process
When buying this lens, I only considered those lenses which had good reviews. Some of the ultrawides have poor write-ups - e.g. the cheaper sigma 10-20, tamron 10-24, etc - poor sharpness.
I limited myself to the following, all of which have their strengths and weaknesses, but all of which are known to be sharp (good MTF):
8-16 sigma: great wide lens, very sharp, extra 2mm (which IS important); slightly cheaper, slightly slower, but you lose the 'normal' 16-24 range. It has an exposed front element - no filter possible - and some physical problems to fit the lens cap - too fiddly and time-consuming. One scratch on the unprotected element and it's an expensive paperweight.
11-16 tokina: no 'normal' range, not cheap (new model?), extra f2.8 capability, but some chromatic aberrations, also 11mm vs 10? It's basically a flexible prime given the short range.
12-24 Nikon: more expensive, smaller range, better build quality, but no improvement in picture, constant f4, but half a stop is not worth worrying about and anyway, most of my pics are 10-14mm where the 10-14 is f3.5-4 anyway, so you're losing half a stop at the end of the lens you're not using and gaining half a stop where 80% of your pics will be!)
24mm is just enough to get those 'normal' pics you wouldn't want to miss while swapping lenses. Really, it's important - and I barely change lenses any more.
On balance, I chose the Nikon 10-24 because it is the most practical and flexible. I chose it because of the wider range, f3.5-4.5, sharp, good results, practical to use - takes filters and standard lens caps.
The pictures are great. Sharp, saturated with colour, precise and really 'feel' alive.
The other lenses will be very good and at various times I've longed for the extra 2mm of the Sigma and the f2.8 of the Tokina, but in the real world (and this is important), I felt that the Nikon would give me what I needed more often. You can't have the best of all three, so pick what's important for you!
The Nikon 10-24 is the best compromise on low light capability vs. flexibility of 10-24. Forget the 12-24 Nikon, I really like that this goes to 10mm - probably 80% of my pics are at 10mm! My second choice would actually be the Sigma because I would love to go wider still, but a second choice because of the range and the exposed element. I wouldn't pick the Tokina because it only goes to 11mm, but I thought about it for a long time and low light might be important (see below).
If you really want an ultrawide, my recommendation would be this Nikon. Mostly because it's sharp, captures real depth and colours, and is the most practical combination.
A note of caution: The older version of the 11-16 Tokina will only autofocus on the higher-end Nikons, so make sure you choose one that works for you. There is a new version 11-16 from Tokina, I believe, which includes an AF motor. You'll need this if you have a 3000 or 5000 series Nikon). The good news is that if you have a Nikon capable of working with the older Tokina, you can probably pick up a brand new older version more cheaply (GBP400?).
A second note of caution (all numbers refer to lens length on a DX camera): None of the UW lenses work with your built-in flash below about 18mm. They display pronounced vignetting below 21mm - this is because the built-in is only useable to about 20mm. When you go wider still (<18mm or so), the lens physically gets in the way of the flash - there's a dark semi-circle at the bottom of all your pictures where the 77mm wide end gets in the way.
I believe that even the best separate mounted flash units will only be rated to about 15mm with some moderate vignetting as you go wider (though I may be wrong about actual useable length), but you absolutely cannot use your built-in below 18mm. The key message is - don't rely on flash when you have an ultrawide, so factor that into your decision if you think you'll need it (Tokina f2.8, anyone?).