I don't use one right now, but I've tried them before. They're a polarizing film that reduces the viewing angles of the screen. I wouldn't say they make the screen completely unviewable from wide angles, but they certainly make it harder to make out whats on the screen. But expect your screen to be dimmer even when you're looking at it straight on and some problems in direct sunlight.
As far as aesthetics, I dont think they look too bad.