Oh, don't get me wrong, Dolby Headphone is good at what it does-virtual 5.1/7.1.
It's just that I just happen to like a lot of the PC games from the time when hardware sound acceleration was common (Thief, Unreal Tournament, and Battlefield 1942 are amongst my favorites), and those tend to sound best on X-Fi cards.
In addition, those games can usually provide proper 3D sound like real life, something more than just a simulated home theater speaker system, due to the fact that the APIs they use pass 3D audio coordinates off to the sound card to mix and play with as it pleases. This is where CMSS-3D Headphone gets its edge, because it can use that information and DH can't.
A lot of newer games don't really utilize the X-Fi DSP as much because of the prevalence of XAudio2 and FMOD Ex and whatnot that do everything in software (and, unfortunately, worse than something like OpenAL with Rapture3D), but then you have the occasional release like Borderlands 2 that actually has OpenAL support hidden deep inside, which X-Fi cards will make great use of.
Also note that a lot of the hype about Dolby Headphone, justified as it is, comes from console gamers who don't really have the option for proper 3D audio to begin with.