I really thought the bigger bikes would be more fun offroad, but I suppose it really depends on the terrain. I rode my Hilltopper on the trails at Windber this past summer. It wasn't bad, but in wet stuff it had absolutely no grip. The TAV really helped grab and climb but the sideways sliding was awful. The Super Broncs are so big that handling them is a real chore on trails.
However, in open fields or flat terrain with little to no curves or turns -- sure, the big tire bikes are great. Nice and comfy on the ole spine. I've heard the Powells are great on trails. With the narrow wheels and the improved suspension, I would tend to believe it.