Taking testing to the extreme

At almost every presentation I make these days someone asks me, What do you think about Extreme Programming? This usually occurs after I have been talking about the importance of design and the effectiveness of testing. Apparently, the assumption on the part of the questioner is that Extreme Programming (XP) downplays the importance of both design and testing. My standard answer is, I have seen Extreme Programming used effectively, and I have seen it fail miserably.