I upgraded from a 20D, and it was well worth it.
Autofocus - Amazingly fast and accurate. From the shots I have taken, I have a much higher % of keepers. Things that should be in focus, are in focus. The 45point system, really seems to capture what you want. It feels like there is more range in the pics I have taken. Shots just look a little cleaner, more dynamic.
Shutter - OMG! You have to shoot with this thing, to really know what I am talking about. Absolutely no shutter lag. The 20D shutter really does feel like a toy, compared to this. The MarkII's shutter feels very precise. A completely different feel, to the 20D. Just an amazing release.
Camera Feel - So nice. My 20D with grip really doesnt compare. Sure, this camera is heavier, however it is more balanced. You dont really notice how heavy it is.
Menu Navigation - OK. This was interesting. It took a good 2 hours before I got the hang of the "hold 2 buttons and scroll" functionality. However believe it or not, its a better system, because when you release the buttons, thats the same as selecting the options. Very hard to explain, however it does feel better. I loved the menu navigation system on the 20D, however the MarkII's seems more thought out.
Also, the fact that you dont have to go into the menu system to change between RAW, JPEG..etc...is great.
LCD Screen - WOW. Detailed and bright. I love it. A great improvement from the 20D.
Hi Speed burst - Amazing. A very fast, precise and clean burst mode.
Overall, just an awesome camera. You can tell lots of thought was put into this camera, when it was designed.
The responsiveness is second to none.
If you are pondering the upgrade from a 10D or 20D and have the cash, just do it. You will not regret it. I promise. Just having this beast in the hands is worth it. For sports or anytype of moving subjects you cant go wrong.
Your % of keepers will be higher. For objects that are more stationary, this camera shines, as you can hold it steadier. Its balanced so well. Its such a relief knowing that you have the best tools for what you are trying to do.