I discovered XP back in 2002, and since then using it (not quite at 100%) for development of the Human Resources Measurement and Evaluation System (HERMES) of Defense.
XP allowed us to discover a new world in developement. Several concepts like TestDrivenDevelopment and PairProgramming are really helping us to be more productive and produce a better quality code in less time.
For those purpose, I developped the REBOLUnit (RUn - http://rebol-unit.sourceforge.net/ ) testing framework, for the practice of TestDrivenDevelopment with the programming language we use, REBOL (http://www.rebol.com).
You can contact me at reboltof