I have the 17-40 and the 70-200 2.8.
At first I'd say the 17-40 is worth it the most because since the day I bought it, it's usually the lens I leave right on the camera.
Then I went from a XTI to a 5D2 and my 70-200 became a more usable lens because of the full frame body. and now I believe the 70-200 2.8 is probably the most worth it even though it costs a ton of money you really can do a lot with that range.
But yeah, my 17-40 is my walk around lens and I do mostly landscape/outdoors stuff.
Same here. Bought mine off craigs list a few days before a vacation for a walk around. Might as well glue it on. Heading for the state fair today. This is the lens I'm taking.

