I liked it a lot. Better than 4 IMO, but then I had relatives in town all weekend and didn't get a chance to read it and finish until 5 AM (read thru all night

). I may change my mind after digesting a bit.
I think JKR does a good job of showing teens and how they act. I've always been impressed at her ability to write "true to age." The characters change as they go through adolescence.
That said, things I liked:
Cho & Harry: He's had a crush for what? 2 years now? It's about time something happened. And I think Cho's emotional insecurities and outburst are very plausible for the situation: She liked Harry, but Cedric asked her out first. Cedric was a hunk and treated her nice. Cedric & Harry disappear, Cedric gets killed and Harry reappears in front of her (and everyone else) with Cedric's corpse. This is traumatic stuff for a teenage girl (or anyone for that matter). I also think a lot of the things that happened to Harry at the end of 4 explain his moodiness at the start of 5 quite well. I also like the idea that their relationship might be over. That's how school crushes go sometimes.
Ron & Hermione. Very believable, more mature but the same people. I wish a bit more of their relationship had been covered, but hey; maybe next book. Them being prefects, Ron on the team, etc. All good to me.
Umbridge. Man oh man! You just HAD to hate her!
Neville. I always like it when minor characters are fleshed out.
McGonagall was cool. She didn't leap off the page at me the way she did for some other people (by the looks of some posts above), but she was cool.
Fred & George. Not for their spectacular exit (& it was spectacular), but for the fact that they took control of their lives and made things happen.
Things I wasn't wowed by:
Dumbledore. I appreciated JKR's attempt to explain some of what he was thinking & doing during the past few years, but it didn't manage to ring true for me. I also have a pet peeve for
deus ex machina characters that seem to never lose. It's like the old cartoon "Voltron"; if the robot shows up, the good guys win: end of story. He's too powerful to be interesting to me.
Occlumency: was interesting to see Snape & Harry one on one like that, but the whole thing was poorly structured to me.
Where the heck was Malfoy!?! I mean, I know he shows up now & again and is snide, etc. But he should've been a major thorn in Harry, Ron & Hermione's side all year. He was a Death Eaters son and a prefect and he was hardly an issue (off the Quidditch field) until "Martial Law" was declared.
I think the sleep-dep is making me ramble worse than usual. I'll cut this off now but I repeat: Great Book! Enjoyed it a lot.