Captivating, breath-taking, stunning are my comments after cramming the book in 5 days time. Though time-killing, I find the book exceptionally enjoyable, even better than the previous four. The style is more or less the same, but with more rhetorical phrases and an improving sense of humor. A wide range of vocabulary is introduced, which means you may have to consult the dictionary frequently if you taste the words bit by bit. Some magical incantations are new, but the meanings of them are not hard to guess.
Comprehensive sketches of characters are introduced. You can actually figure out what sort of feeling each of them had, say miserable, ambivalent, anxious, agitated, and remorseful. In this book, Harry Potter demonstrated an adolescent sensation: immature, curious, rebellious, but kind-hearted. This is exactly the way teenagers react when confronted with what they considered adversities. This contributes to making the book more fabulous and popular.
5th year did not mean extra freedom, but increasing responsibilities instead. Especially when Lord Voldmort was back, the whole wizard world seemed chaotic and confusing. The Ministry of Magic refused all facts, while Professor Albus Dumbledore had to set up the Order of Phoenix against the Dark Lord. Harry, the target of the devils, was to encounter more than he could imagine. Before school, he was attacked by Dementors, mistreated by his uncle's family, interrogated unjustly by the Ministry, and envious of his best friends being nominated as Prefects.
Back to school was no better. Under the supervision of a high inquisitor from the Ministry, nothing was going smooth. O.W.L.S. exams determining his future career had to be taken this year; life-time Quidditch banning imposed by the inquisitor; messed-up relationship with his girl friend; similar nightmares and excruciating pain in the scar he had to suffer from whenever the Dark Lord was rising.