Another one that I missed on the big screen, but with the final chapter (HP7B) coming to the cinema next month (and having already bought a ticket), I thought it would be worth watching the first one, and leaving some time spare between viewings to think about the movie a little.
As with the other movies, they more-or-less assume familiarity with the books, and with the previous movies. In a moment of diversion from the plot of the books, there’s a scene in HP7A with Neville Longbottom, presumably establishing some back-story for him in the sequel.
It’s a really well put together movie (though the endless scenes of camping start to drag after a while), and favourite characters from previous films have a chance to chew up the scenery again. If you’re a fan, you’ve probably already seen it, if not, I’d avoid it unless you’ve enjoyed the books, or, say, HP6.