Well thanks to you lot I went and bought one too!
Arrived yesterday, don't have my headphone amp yet, (Xin SMacro IV) so I hooked it into my loudspeaker system via optical.
Instant, noticeable difference. Not enormous, but enough to be worth while, and I expect to hear a bigger difference through 'phones, as is normally the case with audio equipment.
I did a couple hours constantly A - B'ing with Dark Side of the Moon, with and without the DAC.
The number one thing I got straight away was instrument seperation. Notes are seperated now, I can distinctly here a guitar and a keyboard seperately, without the external DAC they are more blurred together. Allover clarity is greater too, more air around the notes, can hear silence between sounds now where without the DAC it's all one big noise.
I'd say the improvement sounds 5 - 10 % better to my ears, 5 - 10 % more enjoyable
After 24 hours of running there doesn't seem to be any change in the amount of difference between DAC and no DAC, but who can remeber subtleties after 24 hours anyway and this is only through B&W DM604's So no conclusions on burn in or not from me here.
I will definetly be getting well used to the sound with the amp and everything in it's final config. before I start modding it. Which is inevitable I think.