You can't understand without reading the book, but it's a good adaptation.

To everyone complaining this is a "terrible adaptation": It is still a great movie. And to be honest I felt it to be very much in the spirit of the book. Read the book first and watch this one second.

I have no idea how you are supposed to get this film without having read the book first.. But when you have I find it decent, at least when you get over the initial disappointment.