I have a B-68 on the new mtb I bought several months ago. BEFORE I read this thread I was thinking of taking it off because I have a commuter bike and it would do me more good there. I just haven't got around to doing it.
It's very nice, and I like it on the mtb but I was thinking of taking it off because you spend so much time standing when riding trails.
Mud and water won't hurt it. The seat bag protects it from spray.