Voldemort's back. Deal with it.
I started off as the most reluctant of Harry Potter fans. After busting my hump to earn a masters degree in literature, I wasn't about to lower myself to the level of reading a children's book. And the sheer popularity of them did nothing
but put me off further. But then 2001 rolled around and the first Harry Potter movie (Sorcerer's Stone) came out. My best friends were adamant about going--on opening night no less--and never one to be left out, I decided I could break my personal ban on all things Potter for the cause.
While I can't say that I was blown away by Sorcerer's Stone, I can say that I started to get why the stories are so universally beloved. At their heart, they play upon a very understandable, very common fantasy. As many of us can sympathize with, Harry Potter is fundamentally a lonely, unhappy boy who feels like he doesn't belong. Then one day someone comes by and essentially says, "Guess what? You were right. You're better than this. And not only are we going to rescue you from this unhappy life, but in fact you are talented and special beyond compare. And we are going to take you to a place where everyone reveres you and you are very, very loved."
Who can't say that at some point they haven't felt like Harry when he was stuck with the Dursleys? And who can't say that at some point they wished someone would come along who realized they were special? It is the quintessential orphan's fantasy, sure, but one with which all but the most fortunate of us can identify. And when you add the appeal of magic on top of it--well, of course everyone is googoo for Harry.
Needless to say, I have since read all the books, and I have already pre-ordered the 7th and final edition when it comes out on July 21. And I have since seen all of the Harry Potter movies.
In Harry Potter and the Order of the Phoenix, Harry is heading back for his fifth year at Hogwarts after a summer with a conspicuous lack of communication from Ron and Hermione. But trouble is brewing. No one believes Harry's and Dumbledore's claims that You-Know-Who is back, and worse, there is a terrible new Professor of Dark Arts who seems more 
interested in ruining Hogwarts than teaching the kids anything useful. How will they fight the uprising they know Voldemort is staging when the odds are so stacked against them?
Despite the fact that there have been four different directors (Chris Columbus, Mike Newell, Alfonso Cuaron, and David Yates) helming the adaptations of the first five Harry Potter books to film, have now been made into movies, I find the similarities more compelling than the differences. Sure, each director tries to put his own stamp on here or there. But the consistency between the scripts and the actors overwhelms what each man has done.
I find Order of the Phoenix no different. Like the others, it has overzealous script that tries--nobly--to keep as closely as possible to the book but then results in a long, not entirely exciting movie (I find many Harry Potter movies are excellent remedies for insomnia). With admirable consistency, the whole cast is back again. Harry (Daniel Radcliffe), Hermione (Emma Watson), and Ron (Rupert Grint) still aren't very good actors, but the adults of the film all are. In particular, Ralph Fiennes (The English Patient) is getting every more impressive as Voldemort (who knew not having a nose could be so scary?), Gary Oldman (Immortal Beloved) is warmer than you ever thought possible as Sirius Black, and Emma Thompson (Stranger Than Fiction) is still really amusing as Professor Trelawney. And newcomer Imelda Staunton (Peter's Friends, Freedom Writers) is perfectly, pinkly, hateful as the duplicitous, 
wolf-in-sheep's-clothing new Professor of Dark Arts and liaison to the Ministry of Magic, Dolores Umbridge. I will say, however, that I found Helena Bonham Carter's (Fight Club, Hamlet) performance as Bellatrix Lestrange to be over-the-top and disappointing.
Some of director David Yates' camera work is quite impressive: he's into throwing in cool hand-held shots and fancy aerials. And without giving too much away, his handling of the big climactic ending (there's always a big climactic ending in these stories) is gorgeous.
For these reasons, whether you will enjoy Order of the Phoenix relies, I suspect, largely on whether or not you are a Harry Potter fan to begin with. Fans of the books can always find it rewarding to see the stories brought to light in such a faithful fashion. And fans of the movies will find thiss one on par with the others