Harry Potter and the Order of the Phoenix
Imelda Staunton gives an amazing performance as Harry's pink-clad, sadistic new teacher, who manages to remind me of both my grandmother and Hitler at the same time, but the more famous Helena Bonham Carter was a poor casting choice for crazed Death Eater Bellatrix Lestrange. Her performance was nearly identical to her portrayal of Morgan LeFay in the 1998 TV movie, Merlin, but Bellatrix is no giggling witchlet. Carter is not physically intimidating enough for this part, despite her experience with creepy roles, and Bellatrix appears to have no depth whatsoever, which will become a problem in the next film, in which she has a much larger part.
Daniel Radcliffe, Rupert Grint, and Emma Watson have all definitely matured as actors since the first couple of films.
Also, the wizard battle was very cool, special effects-wise, although I'm not quite sure how accurate it was to the books.
In general, it's worth a rental if you're already a fan, but I doubt it'll make much sense if you're not. If you're completely obsessed with the books, you'll probably hate it as well; it does take a lot of shortcuts to become palatable to the general public.