I own a lot of different lens and I really love my EF 70-200mm f/2.8 L IS USM. I use the 2.0x converter on rare occasions to increase my reach, but normally the 70-200 is enough. I have an XT, XTi and a 1D mkII and the 70-200 works beautifully on all three. I'm sure the 100-400 is nice optically, but the push-pull zoom is just not my cup of tea.