Alex King wrote an answer about Mad Rock Mad-Lock Belay Device on June 4, 2010
At the end of the day, this is just a quirky tube device, and does everything a reverso or guideATC can do. The unique feature is that horn that is used to add extra friction when rapping on skinny lines, which would require an extra carabiner for most rappel brakes. So if all you want this for is canyoneering, it could be good since it has the most variable friction of all the tube devices. Otherwise, on multi-pitch you might want a simpler device for belaying off the anchor.