I own every 3.5 non-setting-specific book, plus all 3.5 Eberron books, plus a few 3.5 Forgotten Realms books. It's pretty much all open to use in my game.
It has been a little bit of a problem. I have two players who definitely scour the books for powerful items, classes, and spells. The problem is that since they control only one PC, and I have to maintain and prepare the game-world, they have more time to pore over the rules and find things to benefit them than I have time to realize what they have in mind.
When I run Pathfinder again, anything outside core rules will need a written application (I'm not kidding), with an explanation of why the players wants this rule, how the player intends to use this rule, and how this rule can be exploited. If I'm unsatisfied with the application, it'll be denied. If I accept the application and later find that the player didn't disclose his plans and exploits, I'll take it away.
But I'm a completist. That was true for 3.0, it was true for 3.5, and now it's true for Pathfinder. I'm fighting very, very hard against the completist urge in M&M and Shadowrun.