I am currently playing in co-author Anthony's Rise of the Runelords campaign. We are level awesome, and approaching the endgame. It's been a long haul, and I've been tempted to read ahead in the adventure path books and see where we're headed.
Some of my favorite roleplaying happens when player knowledge and character knowledge are totally disconnected. If you as a player have built (and talked to your GM about) a character development plan that entails being captured, experimented on, and escaping with a newfound level or two in alchemist, it's going to be awesome for you to play through that, even though you know exactly what's coming.
Imagine you told your GM at the beginning of the game: "my ranger never met his dad. His dad is a filthy, blackhearted pirate wizard that left his mom alone to raise their son, and she never told my guy about him." Some time around level 9, when you see a ship coming toward you with black sails filled with arcane winds, you're going to be hugely excited, even though your character doesn't know anything about the scene that's about to happen!
But it's much harder to do that with an adventure path, since odds are the modules don't include a paternally-inhibited wizard pirate captain. But it would be possible to build in reverse: start with something you know is coming later in the campaign, and write something for your character that will make that moment special. I once played a warlock who'd been raised by stone giants. He'd have quite the time in Rise of the Runelords. But going into an adventure path blind, the odds of a character's backstory and their current actions crossing paths are exceptionally small.
Science (Leavitt & Christenfield 2011) (look at that APA style citation, just look at it!) suggests that knowing the end of a story or movie beforehand actually increases the enjoyment you get from it. People who knew the identity of Keyser Soze actually enjoyed The Usual Suspects more than those who went into the movie blind, for example. Weird and counter-intuitive? Absolutely.
I'll bet that finding generalizes just fine to roleplaying games: as long as your players are capable of separating character and player knowledge, letting them know bits and pieces of what's going to happen so that they can interact with that knowledge in interesting ways can improve a campaign. Personally I like giving different players different bits of knowledge, so that their stories can still surprise one another.
Anthony and I will be testing this theory out next year, when I run a Rise of the Runelords game with him as a PC in it. I'm very interested to see how it goes.