When I only used crop, the 17-40 was one of my most used lenses, but once I switched to FF for landscapes, it rarely found a use, except for some more creative ideas, as the corners are simply not good enough on FF, even at f/8-f/16. Mind you, the 24-105 suffers form the same deficiencies between about 24-30mm, but then I now use the 24mm f/1.4 MkII for landscapes at 24mm.
This is why when I eventually go full frame I'll also need to get a better wide angle zoom than the 17-40 I already own, otherwise I could just get a 6D and be done with it. I'm not saying you can't get decent ultrawide shots with the 17-40 on full frame, but especially for landscape use, seeing how the corner resolution falls off even using it on a crop body, I can only imagine how it would be on full frame. Interestingly this is more apparent to me for landscape work at infinity focus than it is for closer subjects.
YES! THIS! I'm noticing it on the 16-35. On close subjects it seems OK, but on landscape shots at infinity (or far away focus) I can't seem to get past the blurry/hazy extreme corners...I almost thought the lens was defective, actually, until I examined some of the shots I took at closer distance....weird.