I've just bought a D1 to replace my Essence STX. I've been pleased with the Essence, but I've just had a new i5/Win 7 x64 PC, and it doesn't play nicely with the soundcard (lots of noise). First impressions were very positive, but for some strangeness: I don't know if it tries to do some weird automatic gain or impedance matching or something, but my first impression was that it sounded nice but had a lot less output than the Essence - especially through the RCA outputs. I have a hi-fi with a Naim integrated amp I sometimes plug into, and through that, it sounded a little bit feeble at first, with not much more drive than a portable MP3 player. Anyway, I've just been out for a few hours, and when I re-booted the computer, it suddenly decided to double its output! Stunning sound though my HD 380 pros, which is what I mainly want it for. If anything, it's a bit too powerful.
So it sounds great, but I think it does have a few technical flaws. You have to unplug the headphones to enable the RCA outs, but I just plugged the cans back in and it didn't enable them again. I had to unplug the gizmo from the computer and plug it back in again. (Seem to remember reading that someone else had experienced a similar thing.)
At the moment, I reckon it's outperforming the Essence by quite a margin, but can't be bothered with A/B tests at this time of night!
I rather suspect that the power supply on this computer is noisy as hell. Tomorrow I'm hoping to take delivery of a toslink cable, and I'll try powering it from a 1000 mv USB mains adaptor instead of the computer. I rather suspect that'll effect a very significant improvement.