What impressed me the most about the D2 though was the quality of the highly regarded audio converters. Throwing a dizzying array of features and a generous software package in is appreciated but if the sound isn't up to scratch, it's all somewhat redundant. It's unclear what converters the U1 uses since the only labelled audio chip inside has Asus' own name on it.
Having the audio converters outside of the electrical maelstrom present inside most laptops will in itself often result in a far cleaner sound with less interference. Comparing the playing of music with my Emu 1616 PCMCIA card does reveal quite a difference, as would be expected. The Xonar's headphone output is reassuringly noise free but the audio playback quality, while acceptable for a budget sound card, noticeably lacks the sharpness and accuracy of the Emu.
Including a microphone in the box is a useful addition if you wish to use a VoIP application. Like the main unit it looks pretty but sounds cheap and rather thin, even when placed close to the sound source (i.e. your mouth). Enabling the array microphone option in the software mixer does remove much of the background noise present but results in an even more flimsy sound. If VoIP is the main motivation for looking at buying the U1, you may well be better with a headset.
In Ubuntu 7.10, basic playback and recording worked although the software mixer behaved unreliably, initially only playing back sound from one speaker and you'll obviously get none of the effects or surround capabilities.
No dice on Mac support though. Just prior to writing this review, I noticed that the Asus website had OSX drivers listed but clearly this is just a tease - trying to download from the site leads to a 404. Should Mac drivers appear, depending on whether the full functionality of the card is available, it could find a very comfortable niche in the more poorly served OSX market. Otherwise the U1 is an attractively designed package with a good range of features but lacks the kind of excitement generated by the D2.
While it looks and sounds reasonable, there's no real stand-out feature on the Asus Xonar U1 USB AudioStation. The USB audio card market is already well served, and quick search on eBuyer shows a USB card for just £11 that has many of the same features.