Read it. As soon as possible.
It's a complete riot. The premise is this: the titular character, Biff, has been brought back from the dead in the year 2000 so he could write a gospel about Jesus' life up until he turned 33 and became famous--pretty much everything that's missing from the Bible.
Turns out that Jesus (or Joshua in the book) had a pretty exciting life before all that other good stuff he did in the New Testament. Him and Levi Who is Called Biff go on a quest to find the three Wise Men who came to witness Joshua's birth, so that maybe they'll give Joshua some advice as to how to become the Messiah. They end up wandering around for 16 years, learning about other ancient religions in Persia, India and Tibet. Then they get back and things get back to what we already know happened. Only, of course, we never got the whole story before.
It's so much fun to read. Biff is the kind of friend I think we all have, an impulsive, sex-crazed smartass who is weak of flesh but fiercely loyal to Joshua. Joshua is also hilarious: Moore makes him completely human and approachable. While he does recite some prophetic wise sayings, he also gets frustrated easily and becomes fluent in sarcasm and creative swearing. He has a great sense of humor even while carrying around the huge responsibility that is being God's only son. Their friendship is at the core of the story and what makes it so great--they're both different but constantly play off each other, and their exchanges are just so damn fun and smart that the story only gets bogged down when the two aren't alone together.
The book is a bit slow towards the start, so it's really when they leave Jerusalem that thins get really good. There's elements of the supernatural sprinkled here and there--they battle demons and Joshua manages to make himself invisible. It's incredibly funny, but also dark and somber in spots, which makes for a completely engrossing read. There's genuinely touching scenes, and the ending (which we all know, really) had me in tears because of the incredible journey the two made together. The best thing, though, is that they're just two guys, one of whom just happens to be the Messiah. They have fun together, they argue, they call each other names and feel embarrassed of one another. They're just two great friends.
I can't recommend it enough. It may take a bit to get used to the anachronisms (of language, mostly, and it's just so damn funny that you don't mind it after a while). Moore is a great writer, and he clearly loves his characters and the story.
And it's worth reading for two things that I loved more than anything in the book(and I loved just about the entire thing): One, the scenes between Biff and the Angel. The latter becomes obsessed with modern television, specially soap operas. The second is the scene where Joshua resurrects Lazarus--just to give you a taste, he refuses to come out of the cave because he's "All icky".
I'm still laughing at that.