The Freakonomics Blog tackles the question. They did make a difference to me at least once. Due to the blurb on Stumbling Upon Happiness by one of the authors of Freakonomics, I decided to pick it up.
The Do book blurbs matter? by Matt Zagaja, unless otherwise expressly stated, is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.