Ah, the good old-fashioned Dungeon Master's Screen.
To its detractors, it is a symbol of DM-player disconnect, encourages out-of-view dice tampering, and fosters an us vs. them mentality between friends. To its supporters, it acts as a shield against the wandering eyes of players, provides the DM with handy charts for quick reference, and fosters an aura of mystery.
I don't really use a screen most of the time. If I really need to conceal a roll, I'd prop up a book in front of the dice tray. Books and notes I have are usually between book pages or face down, and my spot at the table is positioned so that wandering eyes and glances aren't much of a problem.
The only time I've use a DM's screen is as a reference for charts so I don't need to go dumpster-diving through the Core Books.
What about you folks? What are your opinions concerning the screen?