I recently bought one and really enjoy it but if you are only doing paved roads it would be a waste. You lose out to low gearing, rolling resistance and pointless wear on tires. However, my specialized fatboy, with tires pumped up, rolls almost as well downhill as my roadbike before aerodynamics take over.
Its a fun bike, not a fast bike. I like riding it on trails and such because just doing so puts a smile on my face. Instead of avoiding obstacles I now go looking for them.