There are two stories.
Kafka's story and Nakata's story

I pretty much enjoyed Nakata's story. Nakata was an interesting old guy. And also, Hoshino. I can guess Nakata and Hoshino were like comic characters with complexities of their own. But This storyline was interesting and made me read the book.

On the other hand, Kafka's story was full of sheer stupidity. I heard the idea was to tell a story of an adolescent boy. But such boys just don't always think about sex you know! Kafka was trying to find his mother and sister just to fulfill the prophecy his father made? Or just to be with his family again? Even the prophecy was an utter nonsense! There were several times I thought the story would get better it that just didn't happen! Kafka's storyline was pretty much disappointing.

And also, there were lots of hopping here and there from actual story in the Kafka's storyline. Big talks about Arts, Literature, Music bla bla bla..... That doesn't help the story. I don't mean they shouldn't be in a story, that'd make a story pretty much dull; but these things should be short. Not in epic size.

Though it's a single book, I rated it thus:
Nakata: 3/5
Kafka: 1/5