Extreme Programming (XP) was created in response to problem domains whose requirements change. Your customers may not have a firm idea of what the system should do. You may have a system whose functionality is expected to change every few months. In many software environments dynamically changing requirements is the only constant. Does this sound like your last Internet project? This is when XP will succeed while other methodologies do not. A lot of valuable information on XP can be found at the Wiki Wiki powered Extreme Programming Roadmap.