IMO Asus messed up royally when they updated from DS3D-GX 2.0 to 2.5, adding EAX3-5 support. It does this by using very aggressive driver optimization and attempting to appear to programs as if it's an X-Fi.
The trouble is, it lacks several features that X-Fis have in hardware. When it works it's fine. When it doesn't the best case is sound glitches and the worst case is crashing. OpenAL, FMOD and various other current audio APIs have the facility to make use of X-Fi hardware, in some cases automatically. Contrary to popular belief, X-Fi assisted games are still very much alive and well.
DS3D-GX 2.0 actually worked very well at restoring the Xonar's original function under XP. Since version 2.5 they've broken several games that worked fine with version 2.0.