From the Xfire website:
Xfire offers high value, high visibility advertising opportunities to marketers targeting the elusive young male demographic. There are now well over 3,200,000 registered Xfire gamers, and the average user runs Xfire for over 80 hours per month. We offer the only way to reach these passionate gamers while they're actually playing games. Game publishers, movie studios and consumer brands have used Xfire's IAB-standard ad impressions and fully immersive sponsorships to reach their best buyers in an effective, hip, and truly unique way.
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Dunno about others, but the above sounds to me like a really good reason NOT to use Xfire....
Besides, I'm definitely not a part of the "elusive young male demographic", and nobody is ever gonna be able to successfully accuse me of being "hip".