I'm picking up a 17-40mm f/4L and a 70-200mm f/2.8 IS USM. Who has these and how do you like them? I understand that the 17-40 is very sharp and has good contrast on the wide end, has anyone ever found it truly lacking at the narrow end? How is the 70-200 at 2.8, how much better is it at 4 than 2.8? How happy have you been overall with either lens? Any problems with them ever?