Well, there is one strong reason to use an X-Fi (and only one, imo)... positional 3D with headphones. Note that I'm NOT referring to EAX, but to CMMS-3D for headphones which works with non-EAX games like the HL2 and CounterStrike.
For everything except gaming I use onboard sound with digital out, no worries about poor DAC's or amps, sound is as flawless as my receiver can produce. However, IF (and it's a frustratingly big if at times) you can get CMMS-3D working properly with headphones it destroys everything else I've tried (barring Aureal 2.0 with an A3D2 game, imho). It's sad how many people are likely to miss this effect because much of the time you're not getting what you think you are. Just turning it on often isn't enough (for example with Source games, which requires some rather unintuitive fiddling, leaving a huge number of people with a big fat "Meh.")
At first I was thoroughly upset with all the people that had recommended it, but I try to be patient and kept at it. It's bizarre once you finally get it working PROPERLY and you go in game and have that ".... oooOOOOOHHHH" moment.
That said, to me it's still not worth $100. Only got mine because it was dirt cheap used (personally I refuse to buy anything new from Creative Labs after what they did to Aureal.)