Steve Yegge has some good questions about how good is Agile development and how do we know for sure about how good it is. He sees quite a lot of religion in the Agile approaches, something I noticed a lot of while writing Questioning Extreme Programming.
To date we have not done any really good experiments to validate whether the claims of the Agile approaches are credible, sure we have lots of anecdotes, but no evidence either way.