Although I've never bought one on my own, I've come into about half a dozen GM screens designed for use with d20 games. Most of them were given to me, either as gifts, or as door prizes, or whatever. While I used to handwave away the value of a GM screen, I've come to enjoy them. However, I've also decided that my own GM screen needs are perhaps a bit esoteric; I'm not thrilled with what most commercial GM screens offer on the GM's side, and I've also found that the most useful information I've ever seen on a screen was stuff that I put there myself.
The screens that I've acquired over time include the Goodman Games D&D 3.5 screen, with its collection of old school Jeff Dee, Erol Otis (and several lookalike artists) art on the front, the 3.5 screen that came in Dragon Magazine, with the Wayne Reynolds demon-fighting art, the Unearthed Arcana screen that accompanied the Malhavoc Press book of the same name, the Green Ronin d20 Modern screen with James Ryman art, and a fan-made one that a friend of mine sent me; I don't know the actual provenance of that one, though. The friend who sent it to me believes it is the best screen he's ever used; in most respects, I'm inclined to agree.
When I ran my Pirates of the Mezzovian Main game, where a number of the elements that later became DARK•HERITAGE Mk. VI elements were prototyped (including much of the geography) I used five of the six panels of this homemade screen, printed on paper with some Wayne Reynolds Freeport art on the other side. I also created two panels of my own, setting specific information. I then went and had these seven panels laminated at Kinko's and taped them together with clear packing tape.
While in many respects I really loved this screen, in many other respects I didn't. The lamination didn't look as good as I'd hoped. The clear packing tape even less so (plus it created see-through gaps, which my players thought was kinda funny, since it arguably defeated the purpose of a GM screen, at least to some extent.) And seven panels ended up being too many. I found that I could rarely actually stretch it out all the way, so I usually had at least one panel folded in. And the fan-made screen material was too much. I was bombarded with information, most of which I didn't need most of the time. And if I did need it, it would have taken me as long to find it on the screen as it would have in the books anyway. Plus, my home-made screen was supposed to have the added advantage that it didn't cost much (I've always shied away from buying actual screens because $15 or whatever seems like an outrageous investment in something that's so ephemeral)--but the lamination set me back better than $20, making it ironically an extremely expensive option.
Honestly, the most useful material ended up being the setting specific material that I created. Heck, I still pull out the screen with its name-lists sorted by culture when I need to name an NPC., or come up with a tavern name on the fly, and I found the little mini-map of the setting an invaluable tool for the game, which ended up involving quite a bit more traveling than I originally envisioned.
That said, even my brief efforts in creating two panels of setting specific info convinced me that doing layout for something like a screen panel is much easier said than done, and it takes a brave soul to tread into that arena. Or, like me, a naïve one, ignorant of the pitfalls. Nevertheless, I'm tempted to go there again, if for no other reason than because I still don't have a screen that really gives me all that I want and not any (or at least not much) of what I don't. And the setting specific information has been a lifesaver during play numerous times; I'll never game without preparing a few shortcut panels like that again and at least paperclipping them to whatever screen I do use. And if I'm going to create DARK•HERITAGE specific panels, I might as well keep going and create an entire screen of what I want.
What I don't need are endless equipment lists. That's relatively easy to find in the books when I need it (although some equipment is nice.) I don't need a bunch of Skill check DCs, because I can make those up on the fly easily as needed. Those two items alone, especially with armor and weapon details, take up the most real estate on most screens that I have. A much more useful, but much less common section would include a condition summary, so you can tell at a glance what happens if you fail a save and become sickened, for instance. Or what the difference is between being sickened and nauseated; between shaken, frightened and panicked, etc. Because it's DARK•HERITAGE and not generic D&D, the firearms rules would be nice, as would the chase rules. Rules for all the "weird" combat maneuvers (when not using Pathfinder's simplified version) is essential. Lists of poisons and diseases would be handy. Attack and Armor Class (or Defense) modifiers for different conditions would be nice. And maybe I could fill in some gaps here and there with a few skill sections on skills that I don't use that often. Maybe.
I've also got in mind a new construction technique. This time, I'll create a screen that holds up well, looks better (hopefully) and doesn't cost me anything except the ink to print my panels. And, I'll take pictures of various stages of construction. If it all turns out well, I'll even post a step-by-step how-to of what I did and how well it worked out. If.