I used to carry one when climbing in a group of three, in order to belay two seconds simultaneously. I didn't find it convenient for rappeling though, so I carried a figure 8 instead. Now I have replaced both with a Tre Sirius which does everything better.
Pros: Lightweight. It is auto-locking and can be "remotely operated" (i.e. you can walk away from it as long as you hold the brake end). You can belay two seconds simultaneously and independently.
Cons: If one (or both) of the seconds falls, it is very hard to unlock it. They can help by unloading the rope but this is not always easy (and if it was they wouldn't have fallen there).