Running the Right Mark Audio Analyzer utility (which in a sign of its confidence of the card's quality as Asus includes it on the driver CD), gives perfect results. With a signal to noise ratio of -118dB at 192Khz, 24-bit when performing the test according to Asus's detailed instructions, the card delivers on its claims. The RMAA results are not as strong as Creative's X-Fi offerings, which I reviewed exactly two years ago but it's not only about numbers.
As an informal test to measure background noise in a real settings, I use a sample editor (such as Adobe Audition), set the recording input to be line level and turn it up to full. The Asus gives the best result I've ever seen, with a very low background noise level of just -106dB.
Listening to a piece with a wide dynamic range is a satisfying experience with a well-rounded and nicely defined sound. It's a pleasurable experience which is the only factor that matters.. Remember though that if you use Dolby Digital Live to connect the card to an amp digitally, the quality of the converters on the amp you're using will be an important factor and of course Dolby Digital is a compressed audio format, so if quality is important, buy some decent cables and use the analogue outputs.
If you're after a basic home theatre card, there are cheaper options that give decent results, such as the Aureon 7.1 PCI previously reviewed. Similarly, I wouldn't recommend the card for gaming since with only EAX 2, it offers nothing that any other generic card can supply (apart from the quality of audio output of course). With a comprehensive PDF manual (marred by the usual translation mistakes), Vista drivers and a thoughtful design, this is an impressive and capable card. If you're after a solution that offers a high quality home theatre experience or want something for entry-level music production, the Xonar offers a strong package.