My 20D loves the Sigma 20mm f/1.8
I have used this lens almost exclusively since I got it. I find that its sharpness is more than acceptable at all f-stops, all of the respective apertures have their creative uses.
I snapped a pic of my domke f-2 that was about 6" in front of me. The depth of field, DOF, is very thin, razor thin, actually.
I really like it and find that I can experiment with it and achieve postive results.
If you have some shots with fast lenses that display this nifty thin DOF, feel free to post them.
Nikon D700/D300|17-35 f/2.8, 24-70 f/2.8, Sigmalux, 80-200 f/2.8, 16 f/2.8 fisheye,
Lots of flashes and Honl gear.