Audio format support is superlative - another advantage over do-it-all video streaming boxes. Not only are the usual suspects played natively - WMA, MP3 and AAC - but also Musepack, MP2 and the open source Ogg Vorbis. There's wide support for lossless formats too, for the audiophile crew. Flac, Apple Lossless and WMA lossless can all be played, as well as the uncompressed formats - AIFF, WAV and PCM.
And sound quality is good from the Receiver's onboard DAC too. It's actually a touch inferior to the Burr-Brown DAC equipped Squeezebox 3 - music has slightly less definition and clarity at the top end - but it's a close run thing. In back to back tests, the difference was barely noticeable as I pumped up the volume on Newton Faulkner's cover of Massive Attack's Teardrop. It has a good solid thump at the low end, a smooth, listenable mid-range and zingy top-end.
Even a complicated piece of music like Mozart's Requiem didn't faze it and it was only when I moved onto some jazz - from Stacey Kent's excellent The Lyric album - that the differences between the players became apparent. The Receiver has a little less presence; a little less three dimensionality than the Squeezebox 3.
But if you're serious about listening at CD quality, of course, you'll go out and get yourself a dedicated DAC or hook it up to your home theatre amp - neither the Wolfson chip in this player nor the Squeezebox 3 can match the punch, timing and atmospherics of a similarly priced dedicated separates CD player - but for casual listening it's fine.
The two things that could possibly be held against this player are, first, its lack of UPnP compatibility - you're stuck with SqueezeCentre (though there are versions available for Linux as well as MacOS and Windows) - and second, its high price. But even though £279 seems like too much for a system without a hard disk, and one that doesn't do video, consider this: it is significantly cheaper than Sonos' similar system (the Sonos remote alone will set you back this much), and it's just as good. In fact the Duet is, simply put, one of the best bits of audio technology I've ever played with and that, for me, justifies the extra cost.