I have Shimano XT hydraulics on my MTB (Santa Cruz Heckler) and BB7's on my Electra Townie cruiser bike.
One time years ago I was riding the Porcupine Rim trail in Moab, Ut. About the last third of the trail I lost my front brake (leaked fluid) so I had to rely on the rear brake only to finish up the ride. That was a bit of a bummer. The advantage of the BB7's being mechanical disc brakes is that you will never have to worry about leaking fluid/failure during your ride. They work great and are essentially hassle free.

My 2 cents.