[ptitchener]

3205 used two CIMS in the drivetrain last year, and I would definetly agree with what other people are saying here. We could move around fairly quickly, but only becuase our robot only weighted about 70-90 pounds. When it came to pushing, it was almost impossible for us and we were pinned fairly easily.
As for other motors to use, the Fisher price motors have almost as much power as the CIMS do, and so do some of the banebots (the RS-550 goes up to about 75% of the power of a CIM). As long as these are geared low enough so that they don't burn out, then they can do the same job that you want a CIM to do. There are even special gearboxes that make them so that the can mount into a space that was designed for a CIM to be mounted into.