In 4 months the only issues I've ever had with my X-Fi XtremeGamer was not being able to use EAX in Call of Duty 2 and only getting output from WinAmp in two speakers. Creative said Alchemy could fix it, but since it was a Steam game, I never could figure out the right paths. To be honest though, I didn't really even give it the old college try, I installed the game just to see if it would run. However, the latest Vista drivers have since fixed both issues.
Other than those two issues (the winamp was more of a pisser than cod), I'm very pleased at how the X-Fi works.