In general, I thought the film was very true to the book. I hated the book.
I don't know whether you noticed, but there was a lot of hype for the whole franchise, and I wouldn't want you to have the wrong ideas, so I'll briefly debunk some of the things you may have heard. Here's some of the quotes from the back of the book.
Right, on with the painful dissection.
Harry thought. Then he said slowly, "It shows us what we want ... whatever we want ..."Ok, so, I added that last bit.
"Yes and no," said Dumbledore quietly. "It shows us nothing more or less than the deepest, most desperate desire of our hearts."
"So... it shows us what we want," said Harry, and slapped Dumbledore upside the head.
Let's take a look at Ms. Rowling's understanding of the physics of broomstick flight: "`He caught that thing in his hand after a fifty-foot dive'". (At first, I thought that this was exaggeration, but it turns out that this is the height at which this sport occurs, so it's presumably a height which they can all recognize). In this scene, he's racing a ball in freefall, thrown from fifty feet up. To do this, "he pointed his broom handle down". By my reckoning, the ball would hit the ground about two seconds later, except that "a foot from the ground he caught it, just in time to pull his broom straight, and he toppled gently on to the grass". That sounds like an awful lot of deceleration to deliver in a very short time, and presuming that the broom itself is what does the decelerating, I'm not convinced that holding on would be much of an option. In fact, if he did "pull his broom straight" (horizontal), it sounds to me like he'd be cut in half. So, not impressive, but it could all be overlooked, were it not for...
At this point you can probably tell that the game might have one or two practical weaknesses. Ms. Rowling, however, is just getting into her stride. In fact, she can't hold herself back from adding this delightful twist to the design of this fabulous game:
Lee was still commentating.
`Slytherin in possession - Flint with the Quaffle - passes Spinnet - passes Bell - hit hard in the face by a Bludger, hope it broke his nose [...]'.
Yes, "Bludgers". Let's just recap on what the "Bludgers" are.
"[...] two identical balls, jet black and slightly smaller than the red quaffle" (which is "about the size of a football")
"At once, the black ball rose high in the air and then pelted straight at Harry's face. Harry swung at it with the bat to stop it breaking his nose". (At this point, Harry was on the ground, carrying "a small club, a bit like a rounders bat". Lucky for him.)
"`The Bludgers rocket around trying to knock players off their brooms.'"Yes, that's right. Not only are the players hanging by their legs from sticks, fifty-feet off the ground, they have psychopathic kamikaze spheroids the size of watermelons to contend with. And let's not forget the players are already travelling at speeds equivalent to a "fifty-foot dive", remember, but nonetheless, one such player gets "hit in the back of the head by a Bludger" (emphasis mine) hard enough to make the commentator say "OUCH". So, given all that, no, Lee, I don't think it broke his nose... I think it broke his head. Hope that helps.
And then, of course, there's the scoring. For anyone not familiar with the specifics of the scoring of this game, there is one scoring ball, the football-like quaffle, which can be put through one of six hoops (size not specified, but fifty foot off the ground) for a score of "`Ten points every time the Quaffle goes through one of the hoops'". On the other hand, catching the "Golden Snitch" ("about the size of a large walnut [and] bright gold") scores "`a hundred and fifty points', so they nearly always win". Not only this, but (brace yourself) "A game of Quidditch only ends when the Snitch is caught". Now, I don't know about you, but to me, that screams "kill me and dump my body".
"A game of Quidditch only ends when the Snitch is caught". In other words, at the end of a game, one team will definitely have scored a hundred and fifty points. A hundred and fifty points which the other team will have had to make up in goals. That's right. The first *fourteen* goals either team scores are utterly useless, because someone's going to catch the snitch at some point anyway. I mean, I suppose that's fine, because a game could potentially go on for days, if the bloody thing decided it didn't want to come back in the first place. God knows why it ever does come back. But based on the fact that the players and audience actually seem to look forward to this game, let's presume an extremely generous upper limit of three hours. So, the question is, how much might we expect scores to differ by before the Snitch gets caught? Well, given that they presumably spend 99% of the time PICKING THE BALL UP, I'd say probably not fourteen.
Finally, what's the game like from our hero Harry's point of view? Well, Ms. Rowling doesn't tell us specifically, so I've taken the honours upon myself. It turns out that only one person on a team, the Seeker, gets to catch the Snitch, so naturally, that's him.
"Tum-te-tum, seeking, seeking, seeking. Looks like my team are doing well. They've only got to score another, ooh, twelve times to make any difference to the outcome. Seekety-seek. Nope, no sign of it. Probably should have kept an eye on it when they opened the box. Great game this. Tum-t - hang on, there it is. Well, I'll just fly at it in a straight line, try not to get skewered by my opponent who's doing the same thing, and grab it without dicing my fingers... Hmm, that was almost too easy. What's that? It's worth so much that I've won the entire game? The others were basically wasting their time? We'd have done better having the whole team sitting lookout for this thing the whole game? What can I say? Damn, I'm good. Wish they would have given me one of those rounders bats to brain my opponent with when that ball showed up though."
So, it turns out that there's a door (unremarkable in the book, a heavy oaken affair in the film, natch), behind which is a three headed dog, strongly hinted in the book to be Cerberus itself. And the three adorable kids manage to bumble into it. (Apparently due to non-euclidean staircases. "ooh no, we're on the third floor!". WHERE DID YOU THINK IT WAS GOING? TO THE BASEMENT?) And whaddya know, the door it's hidden behind has the kind of lock that can be picked by precocious twelve-year olds. So, in the book, the dog blinks at them at bit and they hastily unbumble back through the door and it's all jolly shocking. This was clearly Not Very Hollywood. An actual Hound of Hell on screen, and it just sits there? We can't have that. So, in the film, the dog goes mental... and naturally, the three kids apply their formidable strength and push the door shut in its face. Very believable. And very effing necessary, given that it can just about push its [tail. ed.] through that door.
Similarly, the book has quite a reasonable scene in which Potter gets a strong hint that one of his wizardly teachers had "a falling out" with it - he sees him bandaging his leg and talking about it. Since this isn't exactly a big action scene, the film cuts this down to something like this:
"look at Snape!!! he's limping!!! the dog bit him, clearly!"Yeah, right. "I got bit by a three-headed T-Rex, so I have a slight limp today."
(Thanks to Vlad for some of the vitriol)