This is truly one of the best Discworld books I've ever read. It's funny, it's intresting, and it's a very good satire on "The Phantom of the Opera". However, it's on the thin line between the really good discworld books (Intresting Times, Feet of Clay), and the excellent books (Carpe Jugulum, Moving Pictures)...
It's hard to pinpoint exactly what's missing here. That's why it's on the line (I would've given it 4.5 stars if I could...). It's extremely funny, but not as funny as Mort... It's ideas are quite deep, but don't run as deep as Carpe Jugulum's... So, in the end, it's not the greatest Discworld book, but if I would've made a 'top five list', it would've certainly be one of them...