The 7D is an 18mp crop-sensor body, which is a nice alternative to the full frame 5D2. For shooting motorsports and wildlife, the extra "reach" or magnification of the crop sensor is very welcome. It has a 1.6 crop factor, which means that i have a 60% bump in the reach for my lenses, so my telephoto lenses can reach even further!
I received the body a couple of weeks ago and have had a chance to take it out for a quick spin. Being a more recent design than the 5DII, I notice some small but subtle improvements in ergonomics and execution when handling the camera. Very pleased with some of the quick access methods to change setting. When you're trying to change settings very quickly to not miss the shot, having quick access is a god-send.
Unlike the XSI, the 7D uses CF memory cards, which is the same as used in the 5DII. No more needing to manage different memory cards. AND, even better, the 7D and 5DII use the same batteries, so I don't need to have different battery chargers, can swap between cameras, have only one set up backups, etc. The similarities between the two cameras has allowed me to reduce the clutter and overhead of different designs.
So far I only have one niggle with the camera (which has been well documented)......the focus squares in the viewfinder have a convenient way of obstructing your compositions. I find them distracting and I haven't learned to "see" around them yet. I sure hope Canon fixes this with a firmware update.
Overall though, I'm very pleased with this body and happy I made the switch. Can't wait to slap it onto the 500mm and start shooting!!! :-)