OK, well thanks to the ash cloud the shipping to a bit longer than expected but it's now finally arrived.
I've been running the card for about 10 hours now and here are my thoughts on how it performs compared to an Asus Xonar D2:
The sound is slightly smoother, slightly more detailed, and definitely less agressive.
I can now listen to music for more than an hour without listening fatigue.
It's not just the highs that are more forgiving, the whole sound is more relaxed.
Stereo seperation on the Cantatis is a bit better than the Xonar, but the difference is small.
Perhaps if I get better speakers, the differences between the cards will be more noticable.

Whether the Cantatis is worth the extra €100 over the Xonar mostly depends on your listening preferences.
If you're looking for the best card in stereo, get the cantatis.
If you can live with the more "upfront" sound of the Xonar, or actually prefer this, then the Xonar is the better choice.
I use the Xonar and the Cantatis side by side in my pc now, and prefer the Xonar's analogue outs for movie watching.
This is not because the cantatis lacks 5.1 analogue outputs, but because the aggresiveness of the xonar actually works out well when watching movies.
It makes movies sound a bit more dynamic.
I drove both cards in 2 channel and watched a movie to compare them properly this way.

If you are looking for a decent step up in stereo sound from your pc, then the cantatis is a fine choice.

I got tired of waiting for an ex demo overture to appear on ebay, so I ordered a new one from hifiheadphones.co.uk
it's on it's way to Holland now :-).
Can't wait to compare it against my Xonar D2.
Will let you know how it sounds on my system, which is made up of a Marantz SR7400 and Wharfedale 9 speakers (9.5, 9.1, 9.CM)

Thanks guys, it's clear now.
I'll pick up an ex demo overture soon and let you know how it compares against my Xonar D2

Fidelio, can you elaborate further on the SQ difference between the Onkyo and the Overture 192? I currently own an Asus Xonar D2 card, but find the highs / treble quite fatigueing. I almost bought the Onkyo from the wavio shop this afternoon, since that card is known for its laid back sound. Now I'm wondering whether the Overture might be the better choice. Is the Overture laid back? I'm very curious how the top end of this card is compared to C-Media based sound cards.