better than the grigri at a fraction of the weight
The smart is the ultimate single rope belay device out there (there's a two rope version coming out soon so you can use it to rap too). It feeds as fast as you can pull rope through a carabiner, it locks off instantly, and when the leader is hangdogging it doesn't kill your belay hand/arm cos it doesn't rely on you maintaining the friction. It naturally pinches the rope tight and doesn't slip.
Buy this over a grigri - its lighter, has no moving parts to get sand in and gum up, and feeding is so much easier. Although, like a grigri, you must spend some time learning how to use it. Just like a grigri, lowering and feeding are more involved than a non locking atc style tube device and the manual is your best friend here.
Check out the picture for its best work so far - 40 foot fall with no damage except to our psyches