I use one when I DM. I use it for the cheesy "DM is mysterious...OOOohhhhh" purpose, but mostly to organize notes, stats, tables, etc. (would use a laptop if I could afford it). I usually roll behind the screen, but that's so that I don't have to reach around the screen to roll, rathar than to keep them secret. I also don't hide maps, since my players are good at ignoring non-character knowledge. In fact, we usually play on a fully-mapped battle-mat (several sheets of paper, a printed 1" grid, and then trace the dungeon/cave/tower with a pen) or smaller battle-mats that we link together as play moves from room to room.