I'm a big fan of the books. A friend of mine is also a big fan and watched the movie. WARNING SPOILER ALERTS BELOW.
The movie is good ONLY if you've never really read the books. Why? Because the movie doesn't really follow the books at all. It's actually quite disappointing from a fan standpoint. The writers didn't even try to follow the series' storyline. Here are some examples:
Movie: Kludd and Soren both get kidnapped by St. Aegolius. Book: Only Soren gets kidnapped.
Movie: St. Aegolius is run by the Pure Ones. Book: St. Aegolius has nothing to do with the Pure Ones, but is taken over by the Pure Ones later on in the series.
Movie: Owls are moon-blinked by being forced to sleep under a full moon. Book: Owls are moon-blinked by being forced to march under the light of the full moon and chanting their names until they forget them.
Movie: Nyra and Metal Beak are the main villains. Book: St. Aegolius is the "villain." Metal Beak doesn't show up until Book 3 and Nyra doesn't show up until Book 5.
Movie: Gylfie's "captor" is an owl named Grimble. Books: Grimble was never even pretending to work for St. Aegolius; he was one of the captured owls the whole time.
Movie: Metal Beak dies while Kludd, in the end, gets away and becomes one of the Pure Ones. Book: Metal Beak and Kludd are the same owl. Kludd becomes Metal Beak behind-the-scenes and doesn't show up until Book 3.
Movie: This plot never really happens in the books. Book: The actual plot is never replicated in the movie.
All in all, Guardians of Ga'hoole WOULD be a great movie...if it wasn't trying and failing to be based off of the series. =/ However it's a good movie if you've never read the books, but if you're a fan of the series then the movie is not worth your time whatsoever.
All that being said....I do love the books. xD