Why no one talks about the 20mm f1.8?
I have and like the Sigma 20mm f1.8
I liked it only a little on my d90, when I got the d700 I liked it more. 20mm on a FX gives you the same field of view as a 13mm on DX, with a field of view diagonal of 94.5
I like it enough that I normally use it as my wide lens. I am more likely to use it stopped down to f8 than wide open, but I think it is fairly sharp from f1.8 in the center and sharp enough in the corners so as not to be overly distracting. I compared it to the Nikon 20mm f2.8, and found I liked the Sigma better. It is sharper (even in the corners). AF was not super fast on either one, but faster on the Sigma. I also find the bokeh quite pleasing.
Also, there is something to be said for the unique combination of a lens which is both very wide and very fast. Gives you photographic options an f2.8 or f4 lens will not
See my plan (in my profile) for what I shoot with. See my gallery for images I find amusing.