I got the 17-55mm f/2.8 IS a couple of month back. It is VERY sharp even judging by L standard and you get both very good speed for a zoom lens and IS on top of that. I have taken pictures in VERY dark rooms at 0,5sec. exposure and they are okay.
Not as big a zoom area as the lens you mention but at a 1,6 crop factor it gives a very usable range. I got it instead of 24-105mm f/4 L because it has more in the wide end which I need for shooting indoors and because of the increased speed. It is as sharp as the 24-105mm f/4 L.
My camera is Canon 350D