Good. Excellent compared to Canon's lower-cost consumer zooms like the 28-80 and 35-80, and noticeably better than most of the third-party offerings. If you want your pictures to look better than the ones your Mom is taking with her $170 point & shoot, get this lens.
Contrast and sharpness are quite good throughout the zoom range. I'm not equipped to perform critical sharpness tests, but slide images under a 4x and 8x loupe and projected big (30" wide) on a screen look razor-sharp.
Rakeback on Absolute Poker Roulette
The magazine reviews mention some minor barrel or pincushion distortion, but I've never taken a picture with this lens in which the distortion could be detected. You'd probably have to take a picture of a bunch of straight lines near the edge of the frame to notice it. (Hint: if your preferred subject matter is graph paper, you might be better off finding another hobby.)