A very good, almost indispensible lens for me. I have owned the 28-70 f/2.8 L on a 20D and loved that lens for its sharpness, out of focus (OOF) qualities and colour. The 17-55 lens, compared, seems a little less glamorous - the shots just don't seem to have the same OOF impact and flare resistance that the 28-70 did. I will never go back to the 28-70L though because of the IS and wider angle the 17-55 offers.
I do wish for a little more length on the long end - even if it was 70mm - because at times I find I need a tighter shot and have to walk in to the subject or crop later on. To change lenses to a longer focal length would defeat the purpose of this being a walkaround lens.
I have loved using this lens where its utility is best - in low light, static situations like museums and church events, where I use the IS and f/2.8. I have never had any worries using the lens wide open for any shots as far as IQ goes. With flash and f/2.8 I can get great and natural looking shots indoors, definieley better than having only f/4.
The focus is fast, and I don't believe after thousands of shots on my 20D that it has misfocussed once without human error.
Compared to holding an L zoom, this lens is lighter and the focussing/zoom is not as well damped, but the lens still does the job.
There is some flare when I take shots with the light source in the frame - more than I would have expected from the price, however those moments aren't common for me. If the situation didn't require f/2.8 or IS, I would use the 10-22 in overlapping focal lengths for shots with the sun/lights in frame.
If there is some CA or purple fringing then I have not noticed it with my everyday shooting.
This lens is my workhorse. It isn't glamorous, but it is sharp and provides better shooting opportunities at normal focal lengths than anything else Canon offers on an APS-C camera.
I am confident that, barring flare issues (which really are minor and can be somewhat managed by your shooting technique), this lens gives me the best quality and utility a zoom in this range can give.