The July'06 issue of IEEE Software
has an excellent article called "Agile Software Testing in a Large-Scale Project". It presents data on what happens when you introduce some XP practices into a large software project; as the authors say, "...this is the most comprehensive data set from an actual project using agile quality practices that has published its results to date." Very worth reading --- it's unfortunately not free, but you can buy a copy
online. I've been sceptical of the claims made by XP's more outspoken advocates; it looks like I may have to admit I was wrong.