There's also a review of the card on the PC PRO magazine website. The site contains some adverts, so I wasn't sure if I should post the link, but here's what the main part says:
"Comparing it with a Creative X-Fi Elite Pro on a variety of music, the Xonar achieved richer audio, particularly in mid-range frequencies. However, while the Asus supports EAX 2 under Windows XP, DirectSound3D limits its abilities with Windows Vista. OpenAL games ran fine, but EAX 2 titles such as F.E.A.R. sounded flat, and the much-vaunted Dolby- and DTS-based equivalents have yet to arrive in any numbers. In contrast, Creative has developed ALchemy, which translates EAX to OpenAL. So, while the list of supported titles is far from exhaustive just yet, X-Fi cards can still provide atmospheric effects for older games using EAX 2 and DirectSound3D.
On paper, and from our tests, the Xonar is very capable and the sound signal is as clear as the specifications suggest. However, its usefulness revolves around how important you consider EAX. Asus has a point in that only a handful of titles support EAX 5, while most still opt for EAX 2 or are moving to OpenAL. However, the Xonar's commendable attributes must be weighed against its hefty price. For many, this will be too much to pay for the extra sound fidelity and surround-processing options."
