In reply to ianstevens:
> What other lenses do you use yours with?
As I think I said, I have a super-wide (EFS 10-22), macro 100 L, and a 70-200 L.
I was replacing a Tamron17-50 f2.8 mid range zoom which gave me excellent service for about 7 or 8 years but appears to be broken. To be fair, it was a very sharp and fast lens and will be missed.
> Just out of interest what was it that made you think it was the lens for you?
The 17-40 L is slower, no IS and less reach than my old Tamron. From shots already it seems just as sharp and very low distortion. I suppose I chose it for image quality and ease of use (it handles v well). I am not too fussed about IS as I never missed it on my old lens with similar focal lengths and, with high quality ISO ability on the latest dSLR's, it's quite easy to push the ISO up and compensate for a slightly slower lens. I think I may miss not having something around the 50-60mm length but, as I said, if needs be I will just walk closer!
The alternatives I outlined above, again. The canon EFS 17-55 f2.8 IS would be the obvious choice as an almost direct replacement for my Tamron but with IS (there is also a Tamron with IS at the zoom range but doesn't get as good reviews as the non-IS version). I just wasn't that keen on the EFS 17-55 to handle, I could see a lot of distortion, and it won't come with me if I upgrade to full frame. The canon EFS 17-85 just doesn't seem to be optically as good and I think I would get frustrated with it, even though it has a great focal range and IS and is relatively cheap (but again won't work on full frame). Looking at canon lenses which will work on full frame, and which won't totally break the bank, the 17-40 L is an obvious canidate but lacks reach maybe (I'll find out), The 24-70 and 24-105 are nice but lack the wide angle. Now I could have coupled one of those with my canon 10-22 but I have been quite disappointed with the results from that (v soft at the edges) so I am treating it as a specialist super-wide and don't want to rely upon it for all my wide usage.
At the end of the day, you need to go out and handle them (not just buy off the internet ideally) and see what you think. The 17-40L felt right at home on the camera and I am sure I will get loads of use out of it. The 10-22 will cover the super-wide angle and I have the 70-200L for anything long. If I am finding I need a real portrait type lens I can consider something to plug the gap but I suspect I will be fine with what I have.