I have been using one, but I developed a dropping problem with it so now I'm back to tape and ultra slide.
For me, my thumbhole had to be really really tight to get a consistant release. Anything less and you were either having to grip the ball, or you were throwing the ball in your teamates laps behind you.
It has a seam that runs down the righthand side of the sock that digs into your knuckle, and it gets annoying when you bowl long tournaments with it.