Have a D90 with 18-105 VR, 70-300 VR and Nikon 50 1.8. I am considering a Nikon 12-24mm (used) but can spend only about 650.00. Image quality in the end is my formost concern. A variety of peoply have suggested non Nikkor lenses stating that I would get very similiar performace at a portion of the cost.
The tamron 11-18 is the bargain lens. The new Nikkor 10-24 is surprisingly (to me) a good lens but pricier. Would I get the 12-24? I found it a bit soft at the 12mm wide end (at 100% crops), but otherwise very good.
I have been using the Tokina 12-24 for a couple of years, and am very satisfied with it. May jupdate it to the 11-16 at some point but no real need to.
I'm very much into ultrawide and have owned three or four different ones. The best value seems to be the Sigma 10-20mm. I owned one of those before buying the Tokina 11-16mm f2.8, which is outstanding. I really need the f2.8 and no one else has it. It also has the least distortion of any of the five or six ultrawides I've tried. That may be because it has less zoom range than other lenses. That's no problem for me since my next lens is the Nikon 17-55mm f2.8. Don't tell Tokina, but I would have paid over $1,000 for this lens and been happy. The only trouble you're going to have is finding one in stock. They sell out very quickly. Note how straight the lines are in this shot. Also, this was a dimly lit scene and the f2.8 saved the day:
