MJ Johnston wrote an answer about Petzl Grigri Belay Device on February 3, 2010
I've had to use a Grigri for an emergency rappel and it's not ideal at all. If you're looking for something with a little more friction than an ATC or something you can regulate a little more I would recommend looking at both the Petzl Pirana or the Blue Water Rescue 8. Both are great options for rappelling that give you the option to lock off or ad several different levels of friction. Though with any 8 type device, it will twist your rope up which can be a pain.