The book is pretty good. It's an interesting milestone in the series, it's the book in which Rowling really goes deeper into her world at last (the kids understand more and could be told more) and her writing style becomes more interesting for adult readers. Its begins in POA with Sirius, but it really explodes in GOF.

The movie truly is awful. While it's been true to an extent for all the movies vs Rowling's books, the huge problem with this one is that it's truly a characters's book, both for the trio and several others who begin to have larger parts. The triwizard tournament is almost a background storyline but for the few chapters about it that punctuate the book and provide the device leading to the unexpected real climax. They made an action movie out of the tournament and little else (they didn't have much of a choice, it was either that or take a turn with the franchise and focus on the characters rather than the vfx fiesta, not a wise "hollywoodian" choice) and the tournament story is a bit blah if you string it up like this; Rowling had designed this as something that recurred and unified the school year, but that left her plenty of room and excuses to tell the real story in between each challenge. So yeah, the book is much better.

The other thing about the book is that while the movies have done a decent job at simplfying yet keeping the essence of Harry (especially in the Yates's ones), Rowling's Ron and Hermione are much, much more involving and interesting characters in the books, and have a great deal more importance, with their own character arcs. OOTP is a good movie for e.g., but the book is far more complex and involving.

Also, don't be surprised of two things: the first two scripts were pedestrians and slavish to the books, and beside they are far more childish (on purpose) than the others (the third as well, but far less so already). There's not tons of stuff that didn't make it to the sçreen in some capacity. If that annoys you, read fast (they're really short) and get to the third and the books, with their more gronw-up tone, and there you'll start getting a lot more story, a lot more coherence and a lot more about the main and secondary characters that the movies had. The other thing is to keep in mind is that the movies have taken every opportunity (and created more when there wasn't enough to their taste) to inflate scenes into visually impressive and cinematic scenes. Don't hold your breàth for that sort of thing in the books, Rowling is far less over-the-top in that respect (but a lot, not all, of her version of those scenes are more satisfying, but would not have been very exciting on screen. A few scenes are better on-screen than her book version.