If you haven't read the book, don't even see the movie. It'll be near incomprehensible.
If you've read the book, well... you're probably going to see this anyways, but you may want to just wait or see it cheap, cause it's got problems.
The movie is largely a whirlwind tour of snippets from the book - flash, Luna's in a lion hat, flash Fenrir (never referred to verbally by name or discussed), poof Tonks (ditto), etc. Don't expect substance, don't expect to feel Harry and Hermione's love problems, etc. One awkward moment of shoelace tying and then we're flashing to somewhere else.
There are good scenes, to be sure, but this was still the worst of the Potter movies.