I'm a huge fan of the 17-40L. Mine had to go into Canon twice, though, and they still couldn't get it right. Luckily, micro adjust saved the day.
I've always been thrilled with the colors from this lens, even compared to other L's. I have no actual hard data to verify why I feel this way, but I do. It's especially pleasing with a good CP.
For me to upgrade, I'd need to see a 16-40 IS, or a 12-24 2.8.