Unfortunately, the film fails in every possible way. The acting quality from the opening scenes is downright appalling. The Dursleys, and even Daniel Radcliffe, are just downright dire initially. Mrs Figgs is cringe-worthy and the Dementors look very cheap and unconvincing after the third film's quality.
Despite all the fuss made about the casting of Tonks, she's barely in the film at all. Many of the significant issues that help build up to the final confrontation and its consequences are cut. Some cuts do make sense, others definitely do not (such as the Sirius/Snape antagonism that Harry feels directly contributes to Sirius' fate).
I've always been a fan of Michael Gambon's Dumbledore - Richard Harris pulled off the grandfatherly side of Dumbledore but completely failed to portray the crazier, darker side. Gambon has always pulled off Dumbledore's crazier, darker side very well while still covering the grandfatherly aspect. However, in this film, Michael Gambon, Emma Thompson, Maggie Smith... all their performances fall flat. As do the children. It took me a while to grow into Luna. At first, I thought it was awful. In retrospect, I think she gets better as the film goes along.
The only two who consistently acted well were Gary Oldman and Alan Rickman, but since their roles in the film were vastly reduced, they don't have much screen time. The dreams were hit and miss - the suspense and eeriness were very good but if you haven't read the books, they'll be confusing and not make much sense - and this isn't plot-related confusion.
The film's final confrontations were absolutely great. Ralph Fiennes makes an excellent Voldemort, his scenes were therefore very good.
Bellatrix was absolutely awful. She didn't come across as crazy and dangerous, she came across as drunk. It was less Harry Potter and more Practical Magic. Not good.
Everything felt disjointed, rushed, superficial. Too much got left out and what was left in was handled poorly. The end result is a film that looks like it's full of plot holes, continuity errors and lacks any emotion at all.
I knew it was a bad sign when the people leaving the screening before my showing were looking bored and restless - few smiles, little conversation about what they'd seen. No real interest. When I left the cinema after my showing, the word I heard my fellow cinema goers using most was "empty" - the film felt empty. I use the word "hollow". This film lacks any kind of a soul.
There's a terrible thread of Charlie Chaplin/Chevy Chase-esquire humour running through this film that is absolutely inappropriate to the storyline. Some humour is appropriate and genuinely raised some chuckles in the cinema. But the rest cracked no smiles and often some embarrassed shifting in the seats from people. For a story that's getting darker and grittier as it progresses, watching Filch and Malfoy reduced to Charlie Chaplin and Chevy Chase comic pieces was downright appalling - and ruined the mood of the film.
Oh, and the forecasts in the background at the start of the film bizarrely give out American weather temperatures. Why wasn't this caught? It's a British story, temperatures should be given in Celsius. It stood out like a sore thumb (British forecasters do sometimes *additionally* give Fahrenheit temperatures for the older generations but never at the expense of forgetting Celsius).
Lupin's role was vastly reduced in this film - I have no idea why they removed him from the initial rescue group and gave all his lines to Moody - especially as the film doesn't bother explaining why Harry is so comfortable with this Moody after all his problems the previous year. The book used Lupin to ease the transition for Harry, but he doesn't appear until much later - and even then is barely relevant to any scenes. Tonks and Kreacher had blink-and-you'll-miss-them roles and Sirius' family lineage was hardly explained at all. There's absolutely no hint of Tonks' relationship - you see her mother's scratched out name on the family tree in Sirius' home, but no reference to Andromeda being Tonks' mother. Therefore, if there's going to be any speculation in the sixth film about Tonks grieving for Sirius, it's going to come out of the blue to people who haven't read the books.
On a more positive note - the film does get better as it goes along, although that just takes it from "mediocre" to "okay". There are some great scenes, there is some great humour and there is some very good acting - but it's so haphazard and patchy that the film overall cannot be rated much above "terrible" - at best, I can say it's the worst film so far.
The scenes where the children are practising DAtDA in secret are very good - the patronus charms are very beautiful, and there's some interesting Harry/Ginny foreshadowing. The much-touted kiss between Harry and Cho lives up to expectations. Considering the youth of the actors concerned, it is set up very well.
Overall, this film is a huge disappointment - even if you go in not expecting much (and I wasn't expecting much at the outset). If you haven't read the books, this film will be a mess. If you have read the books, you'll only be aware of just how big a mess the film truly is.