Yes, it is. I have never seen a movie that has beaten the book (provided that I had read the book). But there are still movies, based on books / adapted from books / etc., that are really good.
Take Peter Jackson's Lord of the Rings Trilogy for example. The books are incredible. They arguably make up the greatest trilogy of all time when it comes to books (if you can think of a better trilogy... well, The Hobbit should count in this as well... let me know). But the movies were fantastic. Did they omit things? Of course! There is no way they could have fit everything in (I missed Tom Bombadil and the scourging of the Shire should never have been cut). Did they add things or dwell on things more than Tolkien did? Yep, and they were criticized for it. But the movies still were amazing.
Now, the Harry Potter movies have not come close to an "LTR" standard so far. Chris Columbus did great with what he had for the first two. The movies were still a gamble and they were doing two at once. He had 2 advantages over all the subsequent movies however. 1) The first two books were short so it is easier to fit things in (even though they still left out, what I thought was, vital information). 2) Two words: Richard Harris. He IS Dumbledore. Seriously, when I read the books, I actually pictured Richard Harris. He played him perfectly (wise, foreboding, humorous, low-key, intimidating, eye that seem to see into your soul yet they always find something they like).
The next few movies saw a change in feel. The replacement Dumbledore was (to borrow from Lloyd Bentsen) no Richard Harris. Now, Michael Gambon is a fine actor (Open Range, Life Aquatic, The Good Shepherd), but from the very first scene he seemed to veer so far from Richard Harris' portrayal that Dumbledore was not recognizable. He seemed to freak out. He was jumpy. He was creepy to some extent. He just seemed to overact a little. But again, I am comparing him to Richard Harris.
The 3rd-5th movies have been... okay. The problem is there has not been much consistency in the look and feel (I know, the books got darker so the movies did too, but the simple geography of Hogwarts was different in each movie... where exactly are the Whomping Willow and Hagrid's house?) and they have left out soooo many important facts that there is bound to be confusion in the movies drawn from the last two books (which, I guess is going to consist of 3 movies... 7 books / 8 movies... okay, but the 6th book should be the one with 2 movies... how much camping can they include from book 7... lol). Yes, the books got much, much longer. Yes, it is impossible to include everything (they tried to include a lot of extra stuff in the 5th through the floating Daily Prophet headlines... decent effort). But, the movies will never be able to do what the books can do.
All that being said, I am excited for Harry Potter and the Half-Blood Prince (watch the trailer!). This was such a good book. For those who don't know (and I don't know how you couldn't at this point) the ending is unexpected (well, it was for those of us who aren't total nerds and sit around and analyzed every little thing in every book so as to predict what is going to happen... I just don't see the fun in that. These are the same people who try to find where their parents hide the Christmas presents).
Oh, two things: 1) the picture above is of Dumbledore meeting Tom Riddle (aka>Voldemort) for the first time when Tom was a child in an orphanage. Dumbledore is giving him the news of his identity and that he will be attending Hogwarts. There is a sense that this kid is severely disturbed even at that time, and 2) the trailer was available at 9pm tonight... I have two exams to study for and a paper to write, yet this post will be up before 10pm. I had to watch the trailer 3 times and write this post or it would have been up sooner. Enjoy!