The first two films definitely take what many would consider a "purist" approach to adapting them to film. Most of the dialog is taken straight from the book, and very fiew important details are left out. However, they lack a lot of the emotion of the books.
I actually get a lot more out of the audio version of the books read by Jim Dale than I do the first two films. Part of it is all the young actors, who are just starting to get the feel of their characters and their acting by the time the third film roles around, but it's mostly the fact that it seems they're just reading the book on screen.
I've always said that this is not at all what makes up a purist adaptation.
The first two films were Chris Columbus trying to directly translate the books to film, getting every detail he could in there. The third film, was Alfonso Cuarón taking the parts of PoA that he thought were most important, and making an excellent film out of them.
Yes, I missed the Quidditch, as I think that element of PoA was probably the happiest Harry is going to be at any point in the series. However, if it wasn't going to be the focus of the film, it simply couldn't be done justice, so I'm fine with the way it was done.
Yes, I missed the exposition on the Marauder's Map, as that's just cool backstory, and yes I missed the discussion of James being Prongs, and Harry's Patronus being a stag similarly.
However, what Cuarón picked as his story was Harry's relationship with Lupin and Sirius, his fight with his own fears and the dementors, and also the enjoyable bit of plot with the Time Turner. He took those elements, pretty much threw the rest out, and made HIS movie, but with full respect and no contradictions to JKR's Hogwarts.
Is it the whole story of PoA? No, but that's what the book's for. It's a wonderful movie though. Harry, Ron, Hermione, and their friends all seemed "real" for the first time. I particularly like the scene when they first arrive at Hogwarts and their eating the candy, and Harry's joking with Ron as he's having nightmares... pure teenager moments.
I hope Cuarón comes back for OOtP (he only didn't come back for GoF because PoA wasn't finished), as that book will need rampant cuts to make a reasonable film.
Does all this make me a revisionist for PoA? I don't think so. It's my favorite of the books so far, and also my favorite of the films. The film didn't take my favorite bits of the book, but it did take just bits of the book, not bits of the screewriter's and director's imaginations... It didn't interpret the magical world or all its creatures the same way I did, but it didn't interpret them in a way I could call incorrect.
This is where I disagree with the revisionists for LOTR so much. They say that's what PJ's films were... his interpretation and adaptation. I say that's true, but he got parts of it just wrong. They don't match up with the Middle Earth that already exists. Cuarón's film is his interpretation and adaptation, which I think is fine, he just didn't screw up any of JKR's creation to make it.
Edit: Try some of these links:
http://www.imdb.com/title/tt0330373/trailers