the fiend folio was worth every penny. the few, non-fluff, creatures in the fiend folio were very good. it's write-ups introduced gamers to many great humanoid cultures like Drow and Svirfneblin.
the entries on Githzerai & Githyanki were rich in detail and plot devices, letting them survive to be used highly in PlaneScape.
and the fiend folio gave us little jerky bastards like the Jermlaine to teach power gamers that size doesn't matter.