We at 665 use a somewhat different system. We use two pneumatic tires in the back for each side. Then we use an omniwheel on each side in the front. The back two wheels are closer together to give more traction from getting pushed from the side and the omniwheels are really close to the front, giving us a good turning radius. The robot turns like a dream, there is no jumpy problems and it is fairly accurate and we can do some good pushing. Even with those omnwheels.
As seen here:
http://www.flickr.com/photos/60426630@N00/399274777/
The omniwheels are kind of a black blur, you'll have to take my word for it that they are omniwheels.
-Greg