Don't get me wrong, I don't think this is, by any chance, a bad book. My low rating can be easily explained by the fact that I've already read too much Murakami.
I used to like him quite a lot, but come on, doesn't he get tired of writing the same book over and over again? Let me show you the pattern. A simple guy who likes to 1.cook 2.listen to music/read books 3. think about the meaning of life meets an ordinary girl who turns out to be totally extraordinary, which gets her into trouble soon after the guy falls for her. The guy tries to save her from something, predictably dark, but fails. The ending is usually bleak and confusing. Doesn't it all sound familiar to you, experienced Murakami-readers?
So, if Sputnik Sweetheart was the first Murakami book I'd ever read I'd most definitely be head over heels for it right now. But after Hardboiled Wonderland and the End of the World, Norwegian Wood,Kafka on the Shore, The Wind-up Bird Chronicle, etc., Sputnik Sweetheart just falls naturally into the plain old "more-of-the-same" category and never moves me like I expected.