I can imagine that this was a tricky movie to make, with the release of the final book in the series preceding it. Now that everyone knows what's going to happen next and how the series is going to end, it must have been difficult to keep the events in this film dramatic and suspenseful and not make everything too obvious. I found it to be pretty successful in that respect. The cast is at it's best yet, particularly Rickman and Gambon, who doesn't drop the ball he was given with this movie--Dumbledore is as much of a main character as Ron or Hermione were in previous movies--and even Tom Felton does exceptionally well. The entire younger cast has only improved with time, really. Pretty impressive.
It's far too short a movie--too much is left unexplored, I think--and could have used an extra 20 or 30 minutes of character development. A couple of later scenes are highly memorable, but the build-ups to the scenes themselves are overwrought.
Equal parts film and architecture showcase, but it's really not all that bad, in large part thanks to an exquisite shootout sequence in the New York Guggenheim. Naomi Watts drops out of the film entirely before anything is really resolved, prompted by Clive Owen's delivery of one of the most contrived lines in recent memory. She's not a lead, just a glorified supporting character, and doesn't really have a character arc of any significance. The ending, reminiscent of a recent film by Andrew Niccol (albeit from a different perspective), was a letdown for me as well.