I also had the MX1000. I'd forgotten the model name, but that's the kind I was using for the 6-7 years prior to the mouse I'm using now. I'll say that if you ever have to replace that MX1000, the Performance MX is a great alternative.

The main difference is in the button layout. In terms of clickable buttons, you lose two from the top (in front and behind the wheel), and add one to the side (at the base where your thumb rests).

This was what took the most getting used to. On my MX1000, I used those buttons on either side of the mouse for going back and forth in Google Reader stories (mapped to J/K). I've had to abandon that, but it's not the end of the world.

I also kind of miss having the docking station, but on the other hand it meant I didn't have yet another AC adapter plugged into my outlets draining a little power when not in use, and it saved some space from my desk top.