User Review: Petzl William Ball-Lock
This carabiner, the Petzl William Ball-lock is the metal sleeve successor to the plastic-sleeved ball-lock. This is my primary locking carabiner, which I use with belay devices, with munters (it is an HMS, designed to take munters readily, even on double ropes), and many other situations. The action is quick and easy with one hand - just push the ball on the sleeve and twist, then release and it locks automatically (do a squeeze check to be sure, of course, as with all lockers).
I did find in Antarctica that in the cold (-30 and below), it was sometimes a bit sluggish. I attribute this to getting condensation into the sleeve and ball while in the tent, which then froze. Working it several times seemed to free it up. I haven't had the problem at temperatures above freezing. And it has worked very well in a couple of hanging changeovers at rappel anchors. The biggest advantage over screwlocks is that the rope can run against the sleeve and not unscrew the sleeve.
I do find the metal-sleeve version a bit more awkward than the older plastic sleeve version with gloves (the black plastic, which is the equivalent). I wish there were an equivalent to the old yellow plastic version which stayed unlocked until twisted shut - this is more convenient when setting up belays or autoblock backups, but of course requires a deliberate action to lock, where both the black plastic and the current metal sleeve versions lock automatically.