There is many belay devices out there today, and the range of auto-locking devices is also growing. A belay device is pretty much necessary on any rope climb, be it sport, trad or indoors. They are all pretty cheap until you get to the premium section of the shelves in most local stores. Ever since an instance involving a rope burnt belayer, and me hitting the deck, I've always kept an auto locking one to hand.
First it was the GriGri 1, the grip was tight and the device was heavy, we got it after realising the big weight difference between me and my fiancée(at time of writing), if I fell at any speed it instantly meant that any rope slippage was just going to escalate. This only happened once or twice before we found ourselves coughing up eighty pounds for this beast.
After training and practice we got the hang of it, as it turns out you pretty much use it like a static belay device. Simple! Long story short, after much use it got stolen... Yep, some people decide to take what is not theirs. So a trip to the shops landed us the GriGri 2, a much smaller and lighter device was the first and obvious thing that we noticed.
The most annoying thing was having to prise open the packaging, extremely annoying but maybe I'm to much of an oaf, so I just talked it with brute force until it came loose. A full set of instructions explain that this works just like the first one but on a wider range of rope sizes; from 8.9mm to 11mm instead of 10mm to 11mm. My current rope is a Beal Top Gun 2 which is 10.2 mm together with the GriGri 2 the pairing seems perfect, providing smooth 'paying out' motion with less locking then the GriGri1 when paying out slack.
I've heard that the GriGri can cause gear to rip out due to the sudden locking of the belay; so far I haven't found this to be a problem on Trad but my belayer is pretty good at rocking in to give a softer catch anyway. Add this to a multi-pitch climb and I instantly feel safer knowing my light weight belayer can rest assured the GriGri will hold up if they we're to be pulled off their stance.
All in all a good investment, perfect for sport, and even Trad with a good belayer. Light weight and easy to use this is definitely a step in the right direction for Petzl.
To back this up I read an article a little while ago where a climber was injured and his belayer was also hit by rock fall and knocked unconscious the item saving him was a GriGri 1.
A few words of caution: This is not a hands free belay device and should be treated as a standard belay device. And finally this requires appropriate instruction before use it is not Newb friendly.