Done!
To comment on the whole issue of 870 pages being inappropriate for a "children's book", I think it's a bit of a misconception that the entire Harry Potter series is directed at, say, 8 year-olds. It's a pretty common misconception, though, probably because the first book really could be said to be directed a a precocious 8 year-old, but that's become steadily less true as the series has progressed.
My take on it is that the "target audience" for each book is, very roughly speaking, the age Harry is in that book: so, 11 in the first book, moving up to 15 in this latest one. This is not only pretty obvious when reading the books, but makes a great deal of sense as the series is at its essence about a boy growing up. This isn't to say, of course, that people of almost any age can't enjoy them, and I'm certainly of the belief that kids can handle much more than what society tends to label as "age-appropriate" for them. But HP:OotP is really not a kids' book.
Quick spoiler-free mini-review:
It's good.
It's not too long, and although it's obviously going to go quicker in some places than others, it's pretty consistent after a slowish start and certainly couldn't be said to sag anywhere.
In a lot of ways it's classic Harry Potter: it has the same structure all the books have been falling into. The biggest difference is probably Harry himself, who is very much the moody teenager, particularly in the first half of the book. This book marks a move even further into Buffy:TVS metaphorical terrain, where (for those who didn't watch the show) the fantastical struggles of a teenaged hero fighting the forces of darkness arrayed against him/her stand in for the struggles and tribulations of solipsistic teenagers growing up everywhere. This is particularly borne out as the plot focuses on Harry's innate psychic connection with Voldemort, and the lack of self-control this brings out in him. Along with this, as with all Manichean sci-fi/fantasy epics, there's an increasing emphasis on the temptations of the dark side, ala Star Wars. Of course, these themes were set up in the earlier books, but here they resonate much more with Harry's personality and his stage of development. On a similar note, all the magical joys and surprises of the first couple books, along with their losses of innocence, could perhaps be said to mirror the development of an 11 and 12 year-old, but I found the connection much more obvious here.
By the end of the book, it would seem that just about all of the filling-in-the-historical-record that consumed especially books 3-5 is pretty much over. Unlike the previous books, there isn't really a huge guess-who-the-real-bad-guy-was-all-along twist at the end, although the plot is still of course rather twisty. The climactic fight is a little dense, what with about 50 pages worth of wizards diving behind furniture and yelling "stupefy" and "petrificus totalis" at each other, although the magic at the end of it is pretty badass. But the really good plot twists come in the denouement, which IMO is pretty great. Of course all Harry Potter books have this end structure: the plot twists that reveal who the bad guys were and what they were up to; the big fight; and the denouement in Dumbledore's office. So, in this one, the bad guy plot bit and most of the fight are somewhat sub-par, but the end of the fight and the denouement very good.
The bulk of the book is quick moving, quite funny (although perhaps not quite as funny as some of the others), and a generally good addition to the series. It's hard for me to say how it ranks with the others, as it's been a couple years since I read them, but overall I'd put it more with my favorites (#s 1 and 4) than my less-favorites (2 and 3), although probably not at the very top of the list.