This has been a significant upgrade from my 300D. I'm loving it and very happy with the move up. It's much easier to work the controls compared to the 300D. I really like the wheel thing.
The only complaints I have and these are mentioned just to be thorugh as I don't feel like they affect functionality:
CF door is shoddy, I guess everyone knows this now but I think we should be driving it home to Canon because aparently the 5D is still the same. I don't think they get it that when you pay over $1000 for a camera the door shouldn't creak and move under your palm. It just feels shoddy and the rest of the build is so good.
People complain about mirror noise but they have to remember that they're getitng 5 fps and you have to move that mirror quickly to get that. I think Canon should have a slow mode that's quiet for single shot use. That would have made perfect sense and quieted all the people who find the noise unacceptable. For me, I don't mind it much at all. I love the sound of it firing - it makes me feel just so "pro".
Unless you're going to spend a lot more money I'd highly recommend this camera. I'll keep it a few years and then keep on moving up the food chain.