Open for 70-80% of shots because of sighting advantages, and decreased possibility of the cue sticking to my bridge, which can be enough of a distraction to really hurt my accuracy of aim and speed. I use a closed bridge for shots where I want to put extra spin on the ball, which for me means force-follow, draw from fairly far away, or stun from really far away. For some reason I can't quite explain, the ball spins more when I hit it firm with a closed bridge, rather than open. I know it's not the bridge that's juicing the ball, but rather the bridge causes me to do something else differently mechanically speaking, but it works for me so I don't question it.
-Andrew