On the Shockwave Radio Theater scale of 9 to 23, where 9 is low and all SKZ Brust works get a 17, I'd rate Harry Potter and the Order of the Pheonix with these conditions:
If you've read the books and seen the previous movie: 17.
If you've seen the movies: 14.
If you haven't seen any of the movies or read any of the books: Don't bother going.
Non-spoiler non-comment: There is nothing after the credits.