If you don't have plans to see this movie, you can check the spoilers here and then come back.
The problem with Project X--the main problem at least--is that it's too committed to its faux documentary gimmick to function as a traditional story, but there are enough storytelling elements jabbed in that you can't suspend your disbelief long enough to take it really seriously; it's not funny enough to be Superbad but it's also not real enough to be The Blair Witch Project (or insert another less dated reference). I didn't hate it and actually enjoyed large chunks, particularly the last half hour where it went from having any pretense of grounding to a game of "how far can we go" in terms of the party central to the "plot" raging beyond any imagining of control. I also thought despite my initial statement that a lot of the character dynamics--the main romantic subplot, the friendship bond--worked to a degree, but going back to that same initial point, they're echoes of relationships carefully crafted through writing and acting because they're trying too hard to seem "real" and coming off less so in the process. I thought Thomas Mann was winning as the sensitive guy protagonist, but Oliver Cooper was too "Jonah Hill lite" as his obnoxious buddy, trying to push the envelope to compensate for not having a character arc. I also loved the pre-pubescent security cards and little comedic flourishes like that, but ultimately, this is a movie you can enjoy one time for the spectacle, but won't hold up for repeat viewings or stick with you an hour later.