There’s an interesting Malcolm Gladwell article in the latest New Yorker on the pitfalls of U.S. News and World Report's college rankings. Gladwell's points regarding the deficiencies of a system that tries to be "comprehensive and heterogeneous" and the flimsiness of quality proxies can be applied to medical school rankings as well.
Despite their many shortcomings, however, I do use U.S. News and World Report's medical school rankings on my website because they are more descriptive than alphabetic order in listing where my clients have been accepted.