I upgraded from the 400D to the 7D a few years ago, after first upgrading my lenses. My main reasons were the better focusing system and faster frame rate for motor sports and wildlife/birding. The extra megapixles that help when cropping and the much better high ISO performance were added bonuses. I have recently added the 70-200 f2.8L is ii to my lens collection and am finding a whole new world out there with the performance of this lens. I am still very happy with the performance of the 7D. I still have the 400D and it still gets a fair bit of use
Keith.