The Grigri Belay Device is a terrific piece of climbing equipment. MAKE SURE THE ROPE IS FED INTO THE DEVICE PROPERLY!! Not only does it have strong braking power (the thicker the rope, the better), but because the feed is also so smooth, it makes belaying much less tiring for the belayer. In the event of a fall, it brakes and holds the instant the rope tries to pull through, and does so with very little slippage. Descending a climber takes a little practice, but becomes smooth and sure. (Because the belayer is letting the rope slide through their brake hand and isn't feeding the rope out, as with a standard belay device, a thick glove on the brake hand, to prevent rope burns, is suggested.)
Overall, whether climbing or belaying, the Grigri Belay Device feels incredibly safe and solid (but don't get lazy... be an attentive belayer). It truly does give you peace of mind... as well as a little rest for the belayer.