We used the normal tank drive system in 2006 and 2007. To make it a little easier to control, we hacked off the long joystick arms. It worked alright, but it was still pretty difficult to conrol when trying to manipulate turns. Going in a gradual in high gear was also near impossible!
This year we decided to go with a controller used for racing remote control cars, and it worked amazingly well (photo
here). The guts of the controller were removed, and it was programed to work with feedback from gear tooth sensors in the transmission.
The steering wheel allows for fine control during high speed maneuvers, something that is very useful for this years game. Turning the wheel without throttling makes the bot turn in place, and reversing the throttle makes it behave like a car driving backwards. I think everyone on the team who tried our new drive system agreed that it was much easier to control than the tank style.