If you haven't read the book, you won't understand a lot of what's going on in the movie, although you'll probably be unaware of what you're missing. On the other hand, you might enjoy it more, being unaware of all the changes that were made. Die-hard fans of the books, be forewarned: this movie deviates more from the book than any of the others. They do not do a good job of explaining the horcruxes, or the complexity of the hallows. The final battle between Harry & Voldemort is quite different, & kind of anti-climatic. However, this is Harry Potter we're talking about here, & of course they wanted to go out w/ a bang! The action is non-stop, the effects were fantastic, the acting was top notch. If you're planning on taking kids w/ you, be aware that there are a couple of graphic scenes, which are more disturbing than in previous HP movies. Also, a tiny bit of language (Molly's infamous line to Bellatrix, which I personally love). An unexpected, but pleasant, surprise, was all the humor that the film managed to include. In summary, it was definitely worth the time, but I was hoping that with the 2-part finale, they would have remained more true to the book.
I am a huge HP fan, but was pretty disappointed with this movie. It didn't follow the book well. Events that didn't even happen in the book were added, leaving less time for the things that really did. And several things were changed. Not enough focus on the important memories seen in the book that help to train Harry for his mission ahead. And they missed a great opportunity for a killer action scene at the end with a battle with the death eaters. Good humor, but overall, not impressed. And I hate to say that!