8 pages worth of discussion on MP: http://mountainproject.com/v/edelrid-mega-jul/108052298
Long story short: Many people like it, some don't like it, everyone agrees it's not perfect. It belays a leader or toproper very well. It can be a bit jerky until you dial in the technique though, especially on rappel. You can flip it around to a plain tuber configuration (eg non locking) for rapping, but it's still nowhere near as smooth as an ATC. A couple people reported that the wire loop popped out from the device mid-rappel - not a true safety issue per se, but enormously inconvenient, and could escalate to be a safety issue in the face of objective danger, etc.
I have one. I use it occasionally. It's sitting there in a bin with my Cinch, GriGri, and Alpine Smart. None of them have managed to replace my trusty ATC Guide - they all have a tradeoff that usually makes them less desirable in one (or more) performance categories. I use my Cinch for single pitch sport, and the plain old ATC for everything else (trad, multipitch, ice, alpine, mountaineering, etc).