I absolutely love the author's rendering of Joshua, and Biff is just the best. I love that he's flawed and completely contrary to everything Jesus preached, yet Jesus still loved him... hell, even Joshua is portrayed a just a normal (if highly enlightened and miraculous) guy. That's a message I can get behind.
It also does a better job than any other book explaining Judaism and Christianity... or rather what Christianity should be. Sure, this is a humour book, but it's supposed to be written as a "gospel", and it does a great job of exactly that.
I am also a kind of history nerd and I loved the underlying history lesson... I grew up reading the bible, but I honestly had no idea about the time Jesus lived in... Very fascinating.