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,
Hutch

I have been weighing up between the EF 20mm f2.8 and the EF 17-40mm f4L.

For EF 20mm: Better vignetting, distortion and CA than 17-40.

Sharpness seems about the same on good samples.

Against EF 20mm: Looks like it has more process variation than the zoom, don’t want to get a lemon. Not much difference in weight.

I had more or less settled on the zoom despite normally going for primes but I worry about the distortion as fixing it in PS seems problematic for fine detail. (see parallel post  PS Barrel Distortion Thread)

Anyone using the 17-40 on 1.6X crop and full frame care to comment on how noticeable the distortion is please.

That's good feedback, thanks hutch.

Can anyone using the 17-40 on a full frame DLSR comment on performance at the edges and barrel distortion please?