I love OotP. In fact I'm becoming quite the zealot. I don't think it's bleak or depressing. (Or at least, I don't feel depressed by it. It's left me feeling very excited about the books, and excited to go back and read and pick up stuff I totally totally missed before). It is very brave and honest though. Yes, cruelty exists. yes, even good people don't act like model pictures of perfection all the time. Everyone is a human being. It's easily the funniest book in the series, and the most moving.

From what you say above, I think you'll like it a whole lot.