We use Pathfinder, 3PP Pathfinder, 3.5 and 3.0 stuff (an adapted Warlock among others) a lot of 3PP for both 3.5 and 3.0, and some d20 stuff - our campaign is almost as much Monte's Unearthed Arcana as PF for classes and races.
I use 3.5 stuff on a case by base basis, but not all of it works well with PF because of differences in design philosophy or because it was not that good in 3.5.
It's kind of interesting. I want to use the old stuff, especially the prestige classes, and as a player I trust myself to keep things on the power level of the group. As a DM, I do not trust all of my players to do the same, and the number of potential problems from all the different skill tricks, feats, classes, etc, is something I do not want to deal with. Balance is the most important thing I have to take care of, mechanically, as a DM. It's hard enough with just PF stuff.
...I had something else to say, but I forgot.
I use first-party Pathfinder materials, but put limits on some ARG races and my current campaign doesn't allow gunslingers.
On the GM side of things, I also use Tome of Horrors Complete (Pathfinder Edition) and Green Ronin's Advanced Bestiary (3.5e edition) for monsters, but I use Advanced Bestiary very carefully, because its templates vary in quality and power. Presently the only thing I'm using out of Advanced Bestiary is the Dread Ghoul template, which also made an appearance in one of Paizo's APs (Carrion Crown, I think). I modded the dread ghoul template just a little bit though, because I made dread ghouls carriers of standard ghoul fever in addition to their normal Create Spawn ability. (On that note I'd actually like to make the standard Pathfinder berbalang a carrier of ghoul fever in my game as well--It's already got paralysis and shares attack forms with a ghoul, and looks a lot like a ghoul with wings. I haven't done it though, to remain true to the berbalang.)
My players, generally have access to any first-party Pathfinder rules materials for character generation. I'd also generally allow spells published for Pathfinder in a 3.5e Pathfinder book (such as Gods and Magic) if it still looks like it would be allowed in Pathfinder. For example, I'd allow Defending Bone as a Pharasma spell, for instance. It's mentioned in Faiths of Balance, a Pathfinder RPG-era Player Companion, but printed in Gods and Magic, a 3.5e-era Pathfinder Chronicles book.
But, generally speaking, I don't allow Wizards of the Coast 3.x stuff or Dungeon or Dragon Magazine stuff in my current campaign. I'm quite happy to be divorced from mind flayers and beholders, but I do wish Paizo could get licenses to include converted couerl and deep crow in one of their upcoming bestiaries. (Both appeared in 3.5e-era Pathfinder Adventure Paths under single-use licenses.) I suppose that probably won't happen though because then the bestiary wouldn't be completely OGL.
We still use our 3.0/3.5 stuff even though we use pathfinder now as the core rules and we havent had any problems yet. The way I see it is I have far more 3/3.5 books than pathfinder and would have happily stayed with that when 4e came out, then I found pathfinder and so just carried on. My group do just see pathfinder as 3.75 and were happy with it that way. And we dont feel like we wasted our money on all the 3/3.5 books.
I allow 3.5 stuff occasionally, but as a GM I really want to have to last word on that. My group used some of the enchants found in the ''Magic Item Compendium". But after Ultimate Equipment I ask them to look there first.
I like to use a combination. There are a lot of balanced and flavorful magic items, spells, and monsters from 3.5 that are unavailable due to the OGL, and as a result the game suffers imo.
Typically, for my games that I run, all of Pathfinder is legal (save for firearms, because I don't like firearm mechanics since they would have such an effect on the world that imo it takes away from the fantasy setting) and I allow 3.5 materials when it comes to certain types of magic items, such as magic weapon enhancements, particular spells that aren't already in PF, and some animals/monsters.
I only use the PF classes, as I don't think it's balanced to have 3.5 classes incorporated since they don't get the same progression that the major overhaul of PF gave to the classes we do have. I also think that with the plethora of archetypes there are, you can do pretty much anything.
I also don't allow 3.5 feats, as their mechanics work so much differently because feats assume you won't get another feat for 3 levels, thus making 3.5 feats a lot more powerful. I would make rare exceptions for players who have a very specific idea in mind for something that is unavailable due to the OGL, but that's on a case by case.
Our campaign started soon after the release of the Advanced Player's Guide, so there was not the wealth of pure Paizo Pathfinder material available that there is now. Our DM allowed a fair amount of 3.5 material back then, so it would be problematical to turn around and ban it now.
Next campaign? I think a good case could be made for Paizo only by that point.
As for 3.0 stuff -- only if it is readily convertible to 3.5. There are a surprising number of 3.0 features that do not convert readily to 3.5 or Pathfinder.