I would use the hell out of this for situations where I normally engine brake in a gasoline-powered vehicle. You can basically harvest energy from the change in gravitational potential energy to kinetic energy as you're falling downhill.

One of the things that sucks about my new truck is that 3rd gear is so efficient that I can no longer engine brake on many of the grades where I was able to do so in the past. It's a much less comfortable driving situation to have to brake periodically on the downhill.