Never seen it, but have read all of it. lolgaxe sums up my feelings pretty much to a T about the second half. The first half is by far one of the best manga I have read. To be fair it was also my first foray into the suspense/thriller/psychological genre.
I agree, I have read the entire thing but the second half was just awful.
One of my most favorite manga/books/movies was Battle Royale. I've read the book, seen the movies, and trying to read the manga but they seemed to have gone away with the regular books and turned them into those massive books which totally jacked up with the like 5 books I already own.