Okay, one quick example: Perry, the book's main character, plays a game called Creatures & Caverns, which is clearly meant to represent Dungeons & Dragons. At the start of the book he goes to a game shop, where the owner offers him a new rulebook and a pewter miniature to represent his character. He immediately says yes to the rulebook since his parents will give him the money for that, but tells us the figurines are much more expensive and he'd need a job to be able to afford one. I play D&D, and the last time I was at the game shop the rulebooks were running around $40 and the miniatures usually less than $10.
It's Vizzini's story and he can make the prices of his imaginary game whatever he wants, but that example illustrates my experience with just about every aspect of his story: all of it was just a little off and I couldn't quite buy into it. Not just little nit-picky facts about obscure games, but the way Perry acted and thought, character dialogue, reactions, and interactions, the plot and action; none of it jived with reality enough for me to find it believable or meaningful.
And while the action was fun and at times engaging, Perry's growth and the life lesson we're supposed to learn along with him were just as off as the rest of the book. I might have given it a better rating just for the ride, but that's what ultimately ruined it for me. He's supposedly a more confident, mature, social guy at the end who has learned not obsess about fantasy stories, yet he didn't learn that from any of his interactions in the real world, but from the fantasy one, and we're supposed to learn it too by enjoying this fantasy story. That just didn't work for me.