I just watched this this morning and I loved every second of it. I came into this never having read the book, and not knowing the story, and in this case I'm glad I did because it meant I didn't know what to expect as it went on. I think this is probably one of the weirdest, craziest, most bizarre movies I've ever seen, and I'm very glad that I did see it.
Edit: My rating: Excellent
I definitely plan on reading the book, and it's sequel This Book is Full of Spiders: Seriously Dude, Don't Touch It.