I've been seeing a lot of people comparing this to the Petzl GriGri, but this isn't really fair because this device is not designed to be fully-locking; it actually just holds the majority of the climbers weight.
But I'll compare anyway since I've used both. For sport climbing this works BETTER than a GriGri in my opinion, because it feeds rope a lot easier and locks almost just as well. To feed rope there is no need to unlock the device because there are no moving parts like with the GriGri; you just push it forward with your thumb and pull out rope with the guide hand. (this actually makes it easier to feed rope than with and ATC)
The locking is very effective. My rope is a 10.2mm and I've also used this with 9.8 and 10.5mm ropes, and for these it holds about 90-95% of the climbers weight.
The amount of rope slippage obviously depends on the weight of the climber and the thickness of the rope, but since I never use thin ropes or belay fast-food eaters it always locks up fine.
Lowering for the first time with this thing was a little jerky though. The trick is the push it away horizontally instead of pulling it up vertically, and then there's the "sweet spot" that lowers the climber smoothly. Takes a little getting used to (3 times for me) and then there's no problem with this.
Overall, this device is awesome. The only thing I think a GriGri would be better for is toproping, although the Smart still works great for that. For lowering, both took me about the same amount of time to learn to control properly, so I wouldn't consider this to be an issue.
This also comes in a 2-rope version which can be used for other things including rappelling. (There are 2 of these for different rope diameters).
Here's a URL for Mammut's video that shows how to use the Smart: