We had our 6 motor two speed geared for about 14.5 in high and 5.5 in low. High gear was awesome for getting across the field but it wasn't the best for turning.
(We had a really high traction setup) Unfortunately, we never really encountered a situation where we had to make use of the low gear for pushing anyone
(anyone up for a fight at the SCRRF Fall Classic?). I think in the course of the competition we might have pushed someone once.
I think the just threat of us made people run so we never got into any good pushing fights.
Our robot that drove the best though was for the 2003 Stack Attack game. It was geared for 11 fps single speed with drills (in high) and Chias. The way we set it up (with chains) was terribly inefficient but that thing drove like a beautiful monster.
I believe shifting is a bit overrated but is still wicked cool to pop it in high and almost instantly be at the other end of the field.