They weren't perfect representations of the book. Nor should they have been - movies are a different medium, and I think they actually did a marvelous job of adapting them to the screen. There are a lot of things that I'd have liked to see, but even with time constraints aside I can't imagine how they could represent Tom Bombadil or the Scouring of the Shire (the two major things that I feel were missing) without wrecking the construction of the movie, which had to take precedence. Maybe I could do some nitpicking about the Balrog and stuff, but most of the decisions they made really seemed to be good ones.
Tom Bombadil can be done in the book because generally it takes longer to read a novel than to watch a movie, so he's not a glaring change of tone, and you have more room to explore the world to make it feel large - movies can accomplish that in ways novels can't, through scenery and background conversation or actions, but because going above 3 hours already earns you jokes about being insanely long, they can't explore tangential setting pieces that don't contribute to the plot itself. If they'd done Tom Bombadil, I feel like the audience that hadn't read the books would spend the rest of the movies wondering when he was going to show up again, and that'd detract from their ability to enjoy the rest of it.
The Scouring of the Shire demonstrates how far the hobbits have come, but that isn't really necessary since the compressed time frame allows us to really see that by the end of the battles in RotK. Given that spending half the movie on endings earns you jokes, working in an hour for something that they can already demonstrate visually and don't need to be quite as explicit about isn't really wise. I felt like Saruman's death could've been better done, but I'm not entirely sure how. It felt a little cheesy, but it had all the vital elements the alternate death would need - it's quicker than the Scouring, it keeps things tied tightly together by having it happen at the conclusion of a scene that already had to happen, and so on. Maybe if they'd spent more time showing Wormtongue being abused by Saruman to build it up, and not had the overdramatic tumble off the tower.