Based on a series of children's books I admit to being entirely unfamiliar with,
The Spiderwick Chronicles is a fairly pleasant adventure that, while not particularly original, is fairly enjoyable. The enjoyment one takes from it is surely relative to their age, but I highly doubt anyone could justifiably hate it.
Revolving around a group of kids who unwittingly come into possession of the "Field Guide" of magical beasts found in their New England country house, The Spiderwick Chronicles is a surprisingly solid movie. Led by Freddie Highmore (who has a rather convincing American accent for such a young actor), they are forced to fend off a wide variety of monsters to protect the secrets of the book.
In a setup up more than a little reminiscent of the climax of Straw Dogs, they must defend themselves and their home from the forces of evil. Aside from the help some supporting adults like Mary-Louise Parker and David Strathairn (who I really, really wanted to say "Good night, and good luck" whenever I saw him), the young talent is left to carry most of the movie, and they do a fine job of it. These three young people are clearly more talented than your average child-actors. The film also does a good job with the action, handling it much more ably than others of its type. Other than the flat CGI, the only real complaint one can place is that The Spiderwick Chronicles is almost too effective. It is far scarier than other kid's movies I've seen and, while it most likely won't scare anyone old enough to see a PG-13 movie on their own, it should definitely provide plenty of thrills and chills for younger views.
As far as special features go, the deleted scenes and various featurettes are plentiful, but fairly standard. Casual viewer's will probably not be too interested in "Arthur Spiderwick's Field Guide" or "A Final Word of Advice...", but younger fans enthralled by the movie's action will hang onto every last word of it.
For younger viewers who want some action and can handle mildly scary material, The Spiderwick Chronicles should be a no-brainer. It strikes me as the perfect rental for bored kids on a rainy day who, after seeing it once, will most likely want to own it. Most other viewers would be largely unimpressed, but parents will be happy because there are far worse movies they've had to sit through.