We used a chassis that had 4 traction wheels in the back and two omnis in the front last year. We had great traction because we used custom 3 inch wide wheels with neoprene rough top treads. We decided not to power the front 2 omni wheels because the chain would have been heavy and extremely long (we didn't want to need tensioners). Like others have said, it turns easily, but not at the center. The wheels were powered directly off a AndyMark Gen2 Shifter, so our low gear was around 11:1, so we were very fast, but not that powerful.
Picture of our chassis