I found this zoom Sharp and contrasty in general. The sharpness drops a bit at the long end of the range, but remains very good.
I found it to be clearly sharper than the 28/2.8 prime at f/4, particularly on the edges (tested with 1.6x sensor of the Canon EOS350D).
I also compared this lens with the very good value 50/1.8 II ( at 40mm and f/4.... ok its a big dodgy given the difference in focal length). The 50 mil was very much sharper in the centre and edges of the picture at f/4
Compared to the EFS 10-22, this lens has a comparable sharpness, it also has a small amount of CA on the edges (almost undetectable on the 10-22), and is 120grams heavier. Finally, this lense appears to include a slightly wider tonal range than the 10-22 (particularly on the warm side of the spectrum). This in my view makes the pictures a little nicer to look at, but the difference is a subtle one and may depend on the lighting conditions.
It also comes with a hood and leather bag (unlike the 10-22).
I intend to use it as my main lens, and maybe to combine it with an EFS 60/2.8 macro or 50/1.8II when travelling (where weight is an issue and you cannot bring 5 lenses with you)
Overall this looks like a very good lens.