This is probably my new favorite climbing tool. The reason I like it so much comes down to its versatility, namely, being able to use it as a reliable auto-blocking belay, and rappel device. The auto-blocking function works excellently for both uses. Rappelling with the Smart Alpine is a little clunky at first, while figuring out how to feed rope through smoothly, but I'll take a little chop from the device catching me over the opposite any day. As with any device, you always want to keep a hand on the brake end of the rope, but it's nice having the peace of mind that, should anything go wrong, you will likely be fine.
One note with this device, however, is that you want to be careful to select a good 'biner for it. Any kind of broad radius HMS carabiner should do. Make sure you set it up so the device is on the broad end of the biner (narrow end facing you), with the spine (opposite the gate) on the same side the rope is running through. The reason for this is that, while it is unlikely, with a smaller carabiner, it is possible to catch the corner, preventing the rope from binding enough to properly auto-block.