Opinion seems to be mixed with the 17-85mm. Some people report softness at the extremes of focal length, some have colour issues and some find that the image quality isn't up to scratch. I've never used it myself, but I know people who have moaned about the general quality of a £400 lens.
You could always shop around for the EF-S 10-22mm and should be able to find that for around £400-£450 depending on age. I had one when I had a 20D and found it to be a lovely lens, very sharp and not that far away from the optical and build quality of an L lens. Bonus is the 16-35.2mm effective focal length, downside is the variable aperture.
However, the 17-40mm is a cracker. If you can afford it (someone was selling one in MM recently...) then go for it. I doubt you'll question your purchase.
