I saw this (as an adult) when it first came out, not having read the books. I'm a huge animation buff so of course I had to see it. As everyone knows and has said, the animation is truly stunning. It is BY FAR the most visually beautiful animated film out there (and I've seen almost all, including anime which I love). The one thing that matches it are the Pandora scenes in Avatar. It is worth watching for the visuals alone, and 3D pushes it to an even more amazing level. If you don't usually watch or buy 3D, this one would be the one I'd say to make an exception with.
As for the overall movie quality, I enjoyed it enough to buy it when it was first released, so it wasn't only the visuals that I liked. You can certainly watch it without having read any Ga'Hoole books, but it is more enjoyable if you have. My 12yo and I just finished reading the first 3 books (which are what the movie is loosely based on), because I like to have her read the books before seeing any film adaptation whenever possible. We enjoyed them and she was very eager to see the movie. It was easier to follow this time because of the familiarity with the characters and plot from the books.
The movie takes some big departures from the books, but we didn't mind the changes at all, and felt that some were an improvement, as they were more plausible. The author Kathryn Lasky was one of the Exec. Producers, so I assume that she was involved in the adaptation process. Even though it's apparently based on books 1-3, from the book descriptions it seems to me to be also based somewhat on 4-6 (which is when war takes place).
She watched it again the day after we saw it, and I predict more repeat viewings!
As for age appropriateness, I'd say 8 and up, as it is more serious and intense and doesn't have a cartoony or upbeat feel of most Disney movies but is definitely more action-adventure. But there are some good moments of humor thrown in, largely provided by the bard-warrior Twilight who, for example, sings a particularly funny lullaby.