Thumbs up/down might not be the best for training a recommendation engine, but as someone who just switched his product's rating system from 1-5 to simple up/down, let me tell you: people have no idea how to use a star rating system. I would get people raving but leave a 1 star review, some people would leave a 5 star review and say bad things, some would leave a 1 star review but seem pretty neutral in their review.

A person is smart. People in general are dumb.