I have a Zpacks twin quilt with DWR down in it that I got back in August. My wife and I have only used it about 10 nights, so I can't say that I've thoroughly tested it. The only situation I've been in that saw any moisture near the quilt was cowboy camping in a meadow on a 28F night - in the morning the whole shell was covered in frost. In addition, we both pulled the quilt over our faces to stay warm and breathed quite a bit of moisture into the top few baffles. I've done this many times with a non-DWR-down quilt and the top baffle ends up being pretty flat from the moisture. With the DWR down there was no noticeable loss of loft, though you could clearly see beads of water on the inside of the fabric when held up to the light.
Other than the DWR treatment and the slight increase in weight (850 fill treated versus 900 fill untreated), I'm not sure I could tell the difference between DWR and regular down. It's very lofty, no noticeable clumping or other weird behavior, and it compresses about the same. Who knows how it will hold up 10 years from now compared to untreated down, but so far so good. I'm in the process of getting a winter quilt and I'm strongly considering getting one with DWR-treated down.