First, I think the Harry Potter movies have been pretty good quality so far, especially the third (partly because it was the best source material) and the 6th one. I don't think they're as good as the books, but then again no one does, and there's also a certain element of always preferring your first encounter with the story.
Secondly, book seven really needs to be two films. There's a good reason film 5 and 6 weren't in two parts each. It's because not enough happens. While very good, long books (though 5 can be a little iffy at times) much of them is spent talking and doing the usual school stuff and going though the whole teenage romance stuff, no real action scenes or huge plot points in either to speak of the except for the final.
Book 7 however has a very different pace to all the other books. It's action after action after action, plot point after plot point after plot point. Hell, the second half of the book is basically just one long action sequence with no time to breath (or wipe the tears off your face *sniff*). Cramming all that in a less than 3 hour movie would just be impossible. It's the shortest of the lot since book 3, but the whole thing is quite literally stuffed. It's partly my gripe with the book. Harry Potter has always put characters before story, book 7 doesn't have time to stop for the characters, plus it doesn't have nearly the same level of charm as the others, so much so it feels almost like a book from different series all together. Still a very good book though, and I think it's format could make for the best movie(s).
EDIT: FYI this doesn't mean they're going to get every frigging detail in there. Even with two movies there just isn't the time, and frankly it's just not feasible or right. Movie adaptions of books shouldn't try to copy page for page, it just ends in disaster. They're two different mediums and should be handled differently.