Like Twinky I also recently made the upgrade, though an even bigger jump from me on the 400D all the way to the 7D and certainly for action based photography its a fantastic move. The camera is bigger, but not too bad, and whilst many of its functions can seem complex at first its really mostly a case of reading through the manual a few times and finding where all the normal functions are in the slightly new interface. After that many of the new bells and whistle addons can be disabled if you shoot in RAW mode. The AF is also a different beast, more complex than before, but allowing for much finer control over its function and much of this, again, you can pick up fairly quickly with regular use.