Sharpness is dramatically improved, flare reduced, improved color, and IS added.

I have the 17-55 f/2.8 zoom and love the sharpness, the f/2.8 speed and IS, but hate the huge size relative to the small Rebel camera

Stopping down only slightly improves sharpness as it's already very good

Awesome contrast and sharpness with some vignetting that can easily be corrected.

The EF 24 2.8 IS USM is a significant improvement over the original EF 24 2.8: increased corner sharpness, silent and fast USM focus and image stabilization

This lens is sharp--just as sharp as my 35L, if not sharper