So I've been thinking about this for a while. Every time I see a movie that was based on a book (at least, a book that I've read), I go into it with the understanding that things are going to be different. Some things that work great in books just don't translate well to screen, not to mention that, unless it's a fairly short book, there's usually a time restriction as well. So, my judgment of whether or not the movie lives up to the book is based mostly on two things.
First, does it keep the overall story intact? Most movies do, but sometimes the movie-makers seem to just take an idea and run with it so that, while it might be a great movie on its own, to call it an adaptation of a book is a bit misleading.
Secondly, and perhaps more importantly, does the movie stand up on its own? Would someone who hasn't read the book be able to follow what's going on without getting confused?
Now, since I'm of course judging movies that come from books that I have read, my view is a bit skewed. For better or worse, I do have all the knowledge that came with the books, and I can fill in gaps that the movie might have left. So, a lot of times it's hard for me to say. For example, when I first saw The DaVinci Code, I thought that some parts seemed really rushed, and I wondered if I'd have been able to keep up had I not read the book previously. But since I had, I really couldn't say one way or another; wonder was all I could do.
And yes, this is on my mind because I just saw the new Harry Potter. Honestly, it left me feeling a little "meh," and I'm sure this is specifically because the ending didn't live up to my expectations. I knew going in some of the major changes from the book, and as I said before, I'm not opposed to that. The majority of the movie was pretty good. But the climax just seemed to lack a little "oomph."
Going in the other direction, I also just finished reading The Princess Bride. I must say, I have to favor the movie in this case, but the book is worth reading as well if you can appreciate the humor behind it. The plot of the story itself is almost identical. There are a couple things here and there that are different, but it's mostly the same story, and a lot of the dialogue is word for word. I had to force myself not to skip over the scenes that I know so well from the movie. But the presentation of the story is what's different, and the author commentary is really funny and incredibly well-done . . . considering it's all 100% fiction. I won't say more than that, here's the Wiki page if you're curious what I mean.