I have had my Asus Xonar DGX for few days now and I have had a chance to try it out in a few games.
First impressions are a little bit mixed. On the one hand the card sounds good and the Xonar control panel has a few nice features that work well with headphones (which I use a lot). On the other hand my attempts to use the Xonar to get EAX support in older games has met with mixed success. and has even led to a few crashes. I tried enabling EAX in the following games: Far Cry (seems to work but occasionally crashed), Painkiller (seems to work), Rome Total War (EAX 2 works EAX 3 doesn't), Prey (seems to work), Battle for Middle Earth 2 (says no supported EAX hardware found). I also tested the card in a few more modern games (Alien Rage and Fallout 4) and it works without any problems but neither of those games use EAX. Asus claims that the GX2.5 feature on their xonar cards supports EAX 5.0 so I am a bit surprised it seems to have problems with EAX 3.0. There are very few games that actually support EAX 5.0 so it is really the earlier versions of EAX that I need.
A lot of people recommend a set of modified drivers that claim to fix bugs in Asus' own Xonar drivers: http://maxedtech.com/ so I installed these drivers and I haven't had any crashes since. Unfortunately the creator of these "Unixonar drivers" is very scathing about EAX support and recommends against using it.
So that leaves me with a new sound card that by all accounts gives much better sound quality than my on-board sound (larger presence with richer mid tones and a hint of citrus or something like that) but which only intermittently supports EAX. Given than EAX was the main reason I bought the card I am a bit disappointed. In hindsight I might have been better to buy a card from Creative to be sure of EAX support. At least the Xonar control panel allows EAX support to be toggled on and off so I am going to leave it off to avoid any possible problems except when I am playing EAX games.
On the bright side the Xonar is working well in modern games and it also has one very useful feature. It can map 5.1 or 7.1 channel surround sound into the two speakers of a pair of headphones. This seems bizarre but it actually works very well and gives you a very realistic surround sound effect.