I've been thinking about getting something for mountain biking. My friend has one and has made some cool videos of our local trails. Here's one of me following him (until he clipped a rock wall with his pedal). I kept up for a little while but I was in the wrong gear climbing the hill and he took off. He also has a 29"er which rolls better.
http://www.youtube.com/watch?feature=player_embedded&v=VzcKdjZ3v88
good luck with the dipstick!