Team 492
http://www.titanrobotics.net has been using 6wd with lowered center wheels since 2000 and team 360 (the first team I was associated with ) used it in 1999. AFAIK we were the first as I didn't see any 6wd robots at either San Jose or Epcot, that year. Since then, they have become pretty popular.
Benefits: 100% traction no matter what the contact is (hills, valleys, flats, going over barriers), fluid, low power turns. In 2004 we were able to run 6 finals on one battery. The motors only got "warm", never hot.
Disadvantage: more complex, somewhat heavier, *harder* to drive straight and easily knocked about
: the reason? Because the robot is pivoting on two wheels, there is very little restraining force. That is why 4wd robots drive so straight: they resist mightily any turn! They also burn up the motors...
In 2004 team #492 solved the disadvantages by using active servo to control wheel velocity. We used very grippy tires (3" beadlock) and used torque estimation to limit current draw when in a shoving match (no need to smoke motors!). Finally, for autonomous mode, we used a gyro chip for heading reference (the encoder feedback from the servo for distance) and never failed to hit the target ball - even after being hit by another robot one time!
Typically we aim for a 15-18 fps top speed, but use the servo to limit speed to something less (say 10 fps). We use the servo for control rather than gearboxes. It hurts our ability to push, but, fortunately, 2004 wasn't totally about pushing. A maneuverable robot that was fast & controllable could still dominate.