It is highly regarded. There are quite a few people who really like its price to performance ratio, and also its lightweight build, compared to the tank-like Canon and Nikon zooms of similar range and max. aperture.
I for one love it as I do the 17-50 /2.8 I chose it over the 28-75 only do to the fact that I use the 50D and wanted a wider coverage. In many ways I think it beats the Canons. Sure AF is a bit slower and more noisy and build is not as good but good enough. I have shot mine in the rain rolled it down embankment (While in camera bag) and knocked it into a few things and there in not even a scratch on it. I don't think you can do much better