[said of XP]: People keep searching for a magic bullet. They want something that will make the construction of large correct and maintainable software systems easier. Ideally done by people they don't have to pay lots of money. But the truth is that there is no one answer or methodology that will deliver this. 

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Most of the texts I've read on XP go to great pains to point out that XP is ''not'' a magic bullet.  Doing XP is not easy, and it won't solve all your problems.

In my (admittedly limited) experience, it ''will'' help with quite a few problems you knew you had, some you were just beginning to suspect you had, and some you had no idea about.  All of this is good.

''But'' preventing yourself from reverting to your bad old ways needs discipline and attention to detail - for example, RefactorMercilessly.
Some of the XP practices are just plain weird at first, no way they're easy - for example, PairProgramming.

Added to this, part of doing XP is figuring out, all the time, which bits of XP you might need to change to make it work better for you and your team.  This is not a characteristic of most of the methodologies that are presented (actually I mean ''sold'') as magic bullets.  -- RichardEmerson