I think the thing that made the book for me was that he was, in fact, a complete realist. The book opens with him accepting that he is royally, utterly, and terminally fucked, and is probably going to die sooner rather than later. And then just getting on with it and seeing what he can do to delay that point.
I have to say that I am dubious about the film. Hollywood(tm) has a long and dishonourable history of taking this sort of "Joe Average doing their best to survive" book and completely missing the point, making the central character out to be some sort of superhuman hero.
However, I'll give them the benefit of the doubt, and go and see it anyway, because it could be bloody excellent :-)