I own a 17-40 (on a 300D). I like it, its a solid lens as you'd expect from an L. My only problem is the 40 part as I sometimes find that short for a walkaround lens.
At work I just got a 17-70 (on a 400D). I'm still getting used to it, but as it is something I'd considered I figured I'd run a few comparisons.
First, the stuff you can't take photos of.
The 17-40 is silent and deadly. It finds focus quickly and makes no fuss about it. The Canon has full time manual focus.
The 17-70 is fairly quiet and pretty quick. It's still good, definitely good enough, just not the same as the 17-40.
17-40 1: 17-70 0
The 17-70 is a lot cheaper. Both come with hoods but the Canon hood is for full frame. Small sensor users may consider getting a different hood for the 17-40.
17-40 1: 17-70 1
The 17-70 is smaller which I would consider a good thing, but it really doesn't have the build of the 17-40. Both have a non-rotating front, but the Sigma extends, while the Canon does not. The Sigma feels solid, but the zoom ring is overly tight and mounting the lens on the camera seems to need more of a twist than any other lens I've used. I'll call this one a draw, better build vs smaller size, as the Sigma still has good build and the Canon isn't huge.
17-40 2: 17-70 2