Aaaaaah the ever versatile GM's Screen. Serving the triple function of hiding your rolls, hiding your notes, and giving you quick access to the charts and tables you'll need to run your game. I've used them as far back as Advanced Dungeons and Dragons (where they were a godsend because of THAC0). In time I've bought a few, made more than a few, and even bought the most excellent Savage Worlds Customizable GM Screen.
I don't use them anymore, and here's why...
1) It created a barrier between me and my players. Not just a visual barrier but a social barrier. It sent the message that I am separate from them and not a part of the group. I think that on a subconscious level, removing that barrier also removed some of the player-gm adversarial perceptions.
2) It encouraged either fudging my dice rolls or the perception that I was fudging my dice rolls. Now I roll on the open table, with jumbo dice that my players can read from one end of the table to the other. I no longer need to announce that I have critted somebody. They can read and I privately relish the gasps and outcries. And of course their faces light up whenever I roll a fumble.
3) GM screens aren't always that helpful. I had a friend loan me the GM Screen for Star Wars Saga Edition and it was so crammed with tables and tiny type that I couldn't find anything. There reaches a point when the game system is simple enough that you don't need the reminders, or it is so complicated that a GM Screen just doesn't have the real-estate to print everything you need to look up. I am better off just printing retypes of the tables I feel I actually need into a chair-side booklet for quick browsing.
4) If all of my notes are on note cards, I just have to keep them face down for privacy. If my players intend to peak while I am having a bio-break, then a GM Screen isn't going to stop them either. At least my note cards will be so short and obtuse that the info will be useless to them.
BTW, when I am done with a particularly good adventure, I put all of the note cards and npc cards in a sandwich bag. Very easy storage!
More and more I'm hearing about GMs either coming out from behind the screen or at least making all of their rolls openly. Good! I say "Stop cowering behind your walls and face your players as God intended; with your dice on one side and a bottle of Dr. Pepper on the other!"