I have one of these on my 40D, and I really like it. At the time, I couldn't quite afford teh 17-50 2.8's or the 17-40 f/4.
As mentioned above, I'd go with all the plus points. I have a nice sharp copy and I'm very happy with it. I think I'd only get rid of it if I moved to full frame, having already acquired equivalent focal length L glass.
The variable aperture is a little annoying - but it's not the end of the world for a lens with good IQ, decent build and very portable.