Don't bother with X-Fi. CMSS3D sounds like ass.
I wouldn't use it for music, but for gaming, the imaging is leagues ahead of stereo
for me.
Note that I still play a lot of older games that use DirectSound3D or OpenAL. In these games, CMSS-3D Headphone doesn't make the foolish step of emulating a speaker system; instead, it directly processes every virtual sound source for a more proper binaural effect. You even get height cues (which are sorely missing in newer games that use XAudio2 or FMOD because they pre-mix everything to 7.1 at most and stereo at least before it hits the sound driver)! (And on top of this, some of those games use EAX 3/4/5, so the X-Fi DSP is still needed for that.)
One major example that comes to mind is Unreal Tournament (which uses OpenAL with the help of an Old Unreal patch; UT 2004 uses OpenAL by default). In stereo, the sounds are clear, but all jammed right next to my ears. I don't have a clear image of exactly what sounds are coming from where. I keep flicking CMSS-3D Headphone on and off to compare, but I still get the same issue with plain stereo.
As you say, though, it's very subjective. At least it's optional and not forced on anyone; toggle to taste. (It doesn't help that you can't get a personalized HRTF on the more affordable binaural surround filters; for that, you'd need a $3,000+ Smyth Realiser and a chance to record a good home theater system. In the end, it's still emulated 7.1 at most, not a true 3D sound field...if only we could have it all.)
Now, I won't get into the usual CMSS-3D Headphone vs. Dolby Headphone or X-Fi (Creative, some Auzentech) vs. C-Media (Asus, HT Omega, most Auzentech) cards, only because I haven't owned a C-Media card with Dolby Headphone and don't intend to buy one any time soon. Note that I actually rather loathe Creative as a corporation, but I don't have much choice in the matter if I want my older games to sound as the developers intended (with EAX). Maybe things would be different if Aureal had survived...
Can the Asus cards output the virtual surround modified signal over a digital connection like the Creative one I've been looking at? That would make it easier to add a better DAC+amp later and keep the surround.
I cannot confirm this directly, but word on Head-Fi is that Xonar cards cannot output Dolby Headphone-processed two-channel over S/PDIF, while X-Fi cards can output CMSS-3D Headphone-processed audio over S/PDIF. That would be a shame if Asus really did impose such a limitation.