My 2¢ to USB Overdrive too!
Steermouse is good hacker for only one mouse at a time, not easy to handle two or more mouse. You have to reconfigure every each of mouse when you use it. It's very annoy to someone like me have two mouses at home and another two in two different office location.
On the other hand, USB Overdrive is very good at handle my four mouses without any interfere. I set up each mouse once, then forget USB Overdrive and just use any mouse when I need them. (One mouse for FPS game like COD4-MW, one for CAD and CS4 design softwares, the other two from company cheap mouse for general use)
Only one concern is:
USB Overdrive 3.0.1 is 64bit using 8.7MB memory,
while Steermouse 4.0.1 is 32bit using 2.4MB memory.
I think 8.7MB memory is not that big deal anyway, so I choose USB Overdrive over Steermouse.
Hope this may help you. 🙂