Honestly I think any LOTR fan should be so grateful for those movies. It could have gone wrong in so many ways. At one point it was supposed to be a single movie. It could have deviated completely from the books, it could have been super cheaply made. Remember, the books didn't have a big pop culture presence at the time. Fortunately, Jackson and others involved were really passionate about doing it right, and convinced New Line to take a big risk on it. Maybe they should have changed nothing, but I think that's unreasonable. It's a different medium, where storytelling and pacing are just different, and also we live in reality, and people are looking for returns on their investment.
The Hobbit movies are different story. I don't know if we'll ever know entirely what happened, but from what I can deduce, there were too many studios involved looking for huge returns, and they made big changes too late, including moving expanding from 2 movies to 3, and switching directors.
The Harry Potter movies are pretty faithful, but they remove tons of content. The books get much longer as the series go on, but the movies don't, so 4, 5, 6 are really flying through scenes just to get through the main plot.