What body are you using? If you are using a full frame body (such as the 5D or 1D series) then the 17-55mm will not be compatible.

I use the 17-40mm on a 5D Mark II and I find is acceptable for most landscape work, it's a little soft in the corners on full frame (as are nearly all ultrawide angle lenses).

If you're using a crop sensor body (7D, 50D, 60D etc) then I'd recommend the Canon 10-22mm for landscapes. I used to use it and I loved it.

Ben