I've made a big jump and upgraded from the 300D (Rebel). After 3.5yrs a few times round the clock it still performed perfectly but the release of the 50D prompted the upgrade.
I've tested with 17-40F4L, 70-200F4L and 50mm 1.8 Mk2 and all perform perfectly, no ERR99 codes. Nicely balanced with this range of lenses.
Ergonomics are great, I would say I have normal sized hands and everything falls into reach during your shot composition.
Checked calibration of lenses before starting use, both L's didn't need any tweaking, as I'd hoped, but the 50mm needed 3 points (of +-20 available) back focus to be accurate. To be honest if I hadn't have checked I would never have known.
Focus is very quick and so far its had no problems in low light in places where the 300D could struggle.
High ISO performance is great in low light, very little grain when going up the range.
The rear screen is excellent, the colour and resolution shows off a lot of information and is easy to pan around. Menu system is easy to use and settings all in a logical order.
The flash is not able to see over the lens hood of the 17-40F4L when wider than 35mm and casts a shadow, that said its not a criticism of this camera, it seems to be with them all. With my 420EX (or take the hood off) again its perfect.
A real plus is that it uses all the same batteries/cards/charger as my previous body.
The feature to be able to control virtually all the cameras functions including taking pictures via USB seems very useful. If you want to use a tripod to take product shots or position the camera for wildlife and operate remotely via USB from a laptop. The only things you cant operate are mechanical buttons on the camera..ie changing mode Ap Sh M Pr etc and obviously zoom and manual focus. However you can change the focus point and then autofocus again.
Although Liveview appears to work well, the autofocus method is a little clumsy. You have to use a separate button instead of the half press of the shutter, the mirror flips, screen goes black, it focuses, beeps and the screen comes back on. Prob about 1 sec max. This doesn't worry me as its not a feature I think I'll be using except extreme cases. For me Canon could have left the whole Liveview feature off a camera at this level.
The only negative comment I could make is that is not full frame ...but thats why the 5D exists at higher price tag.
Would highly recommend.
Hope this helps in your hunt for your new camera.