Neat idea. Quite pricey but thought I'd try it. Works well, and slicker to use than a screwgate or twistlok. And it's black- bonus!
I use this with a Grigri 2 and although it is great with a standard belay device, with the Grigri it does take some time to attach and detach the rope. This is mainly because the Grigri has to sit on the small end of the carabiner, requiring it to be moved all the way round past the internal gate, which an be a bit fiddly to maneuver. Other than that, it is both secure and the magnetic latch makes things a lot easier than worrying about a screw gate.
Magnet mechanism is great. Eliminates the chance of forgetting to lock your biner. I feel the gridlock shape is a bit of a hassle. It's great for things like your prussik which don't move that much, but a hassle for your belay loop. I'd buy a more conventional shape for that.