Ignore some of the naysayers who dislike Order of the Phoenix, as it is perhaps JK Rowling's best installment of the Harry Potter series to date. Readers are given a wider vision of the wizarding world, with lots of new locations including the Ministry of Magic and 12 Grimmauld Place; and the consequences of one's actions becomes an increasing focus. That is not to say that the book is faultless; for example, less time could have been devoted to the subplot involving Hagrid's brother, Grawp. However, this is one of a handful of negative aspects to the book, which are outshone by a compelling storyline in which the boy wizard gets a taste of the real world, after four books which tended to follow the same pattern. I didn't like aspects of Phoenix when I first read it; however, on a second reading, I realised that I preferred its story to the usual "there's a mystery to solve, lots of red herrings, the wrong person is suspected of being the villain" etc that we had become used to fromthe Potter books.
With Phoenix, JK builds upon the more adult aspects to Prisoner of Azkaban and Goblet of Fire, and creates a story in which Harry faces government bureaucracy head on, embodied by the vile Dolores Umbridge, who's obsessive adherence to rules and constant interference in the running of Hogwarts makes her a believable, recognisable foe. Umbridge's inevitable downfall from her position of power contains some of the funniest moments in the entire Potter series, and it is a comeuppance that will have readers cheering. Phoenix also reflects the tricky teenage years, when hormones are raging and tempers often fray. Harry's attempts to cope with the events of Goblet of Fire, and his anger at the Ministry of Magic's attempts to use the media to portray him in a negative way, are manifest in angry outbursts, which should be familiar to many readers, and reflect the changes in both the characters and the younger readers.
Order of the Phoenix is the most ambitious of the Harry Potter books to date, and it shows that villains come in a variety of forms, from murderers to bureaucratic tyrants. If you haven't read it yet, buy it now, as you won't be disappointed!