OK... went out and bought the Sigma today after trying it in the store. So far, so good. The focus feels a bit faster than the 15-85 I sold (probably should given the difference in zoom range), but AF speed was not really the problem for me anyway, more accuracy and shutter speed/noise limitations. Anyway, good percentage of keepers on the first day, but have not really tried in poor lighting. will update when I do. Everything else said about the lens appears to be true... extremely sharp in the center from 2.8, very sharp at the edges from 2.8, a little soft in the corners at 2.8... sharp everywhere by f4. good contrast, bokeh and build. colors appear to be a tad warm compared to the canon lenses I've owned, but I like that about the Sigmas, so no issue for me. I think the corner softness at 2.8 at 17 may be a bit of field curvature as well, but I've done no formal tests as yet.