Well I've been listening to it for a couple of weeks now (listening with Stirling LS3/5a V3's). It is very good. I wanted to hear the Naim sound - which I'd describe as 'vivid', everything seems enhanced and forward but without going over the top or adding a screeching treble.
Having said that, i think I prefer my excellent Leema Tucana 2 (more driving/architectural) or Sugden A21a Sig (musical, emotional) - which i think are 'next level' - but I'm yet to try back to back. So I'm looking to upgrade to the Supernait I think. It's down to personal preferences, speaker matching etc - as usual.
Naim Nait XS3 is currently for sale (on other forums eg PFM, and probably with the WAM once i take a photo), and comes highly recommended.