I am using the 17-40 on my 20D (1.6crop) and love it. It is very well built and has a great feel to it. Image quality has been excellent and having the ability to zoom has been a must have. I would love to have the zoom range of the 17-85 kit lens however wanted only lenses that could be used when I eventually upgrade to lower crop factor bodies.
As for barrel distortion, I think it would be unrealistic to expect the ability to take a photo of the ocean for instance, zoomed all the way out to 17 and expect the horizon to be perfectly straight. I would imagine that when zoomed to 20 it would perform similar to the 20prime. I take mostly landscape photography with this lense and on my 20D, I don't think I could give up the 17-20 range.
Good luck with your decision,