klaus hergesheimer wrote a review of Petzl Am'D Locking Carabiner on July 26, 2011
I prefer these for belaying because they are nearly symmetrical. That means I can let it wear on each end, doubling the life of the belay biner. And the key lock makes things easy as well.
I used to use the ball locks, now I use the triact lock. I used to use manual screw locks, but prefer auto lockers for belay biners because one less thing I need to keep track of.