The Pi itself is pretty lame as a robot controller. The
GPIO pins are all 3.3V, and there isn't any analog IO. That's not to say you couldn't build a cool shield/cape/extension board that would give you a lot of robot control functionality, and
people are working on it.
The idea of having my robot be a full-blown Linux box is pretty appealing. I think I'd take the new
Vex ARM9 controller (which also runs Linux) over the Raspberry Pi when it comes to robot functionality, but it's hard to beat $35.
