Great lens

Mostly positive review, really love this lens, ultra wide angle, solid construction (dunno why KR said about flimsy build, my opinion is this is a quite solid lens even though no match to my "all metal-no bullshit" nikon Ai-S).
Silent & full time override HSM focus motor. Bulbous front element makes me feels like superstud wielding my dream lens nikon 14-24/2.8 (of course with some worries about its safety).
Sure sharpness is no match to Nikon 14-24/2.8, heck, currently there are no match yet for that lens, and then Tokina 11-16/2.8 is faster & sharper too than the sigma (but with lots of "easy to fix" purple CA).

So, if you want ultimate UWA sharpness, nothing less than Nikon 14-24/2.8 (FX with D800E it'd be a dream, on DX it'd be like "not so wide but very sharp" 21-36mm/f4).
If you're using high MP DX camera (D5100, D3200) and want to count all the pixels, then better with Tokina 11-16/2.8, just a simple fix with ACR/lightroom then you get a good sharpness image. Just be prepared for manual focus on entry level nikon, luckily now Tokina has released the same lens with internal focus motor.

However, I'm all about wide, so it is Sigma 8-16, luckily I'm through with pixel peeping syndrome now; yes my Nikon D3200 isn't getting all of it's pixels to their max potential, but I'm with it for its excellent dynamic range (important for landscape with UWA). Shoot with 6MP or 13.5MP.

If you want something even wider on DX, I'm thinking about Samyang/Rokinon 8mm FE or Nikon 10.5DX FE or Tokina 10-17 FE then de-fish it ACR or some other PP to get rectilinear image, you'd get even wider but lower image quality in the corners; plus you got to play with FE lens

Problems:

Slow aperture but this is the law of physics & market price of large aperture wide lenses