Just to add my two cents -- I got my One on Tuesday and I've been playing with it nonstop. I figured I'd search the forums to see if anyone said anything about the dedicated headphone amp. I have to say, this phone does sound quite good. It drove my DT880/600s with no problem (volume-wise) at about 60% volume. Obviously a lot of the dynamics were not there, but I just had to see how well it fared. I use my KSC75s at work, usually with my Nexus 4. But since I've upgraded my phone, I've noticed an improvement in the sound! I haven't done much listening without the Beats EQ, mostly because I do like the sound it produces with the Koss. However, it does sound better to me. I use Play Music with high quality streaming (mostly 320kbps). I don't have any high-end IEMs to test it with, unfortunately. I had a set of 3.Ais pending, but that fell through -- allowing me to better fund my One purchase.
Initial impressions about SQ are quite high, though. I definitely notice an improvement over the Nexus 4, which was decent, but nothing special.
Other general impressions: obviously gorgeous design, speedy interface, and damn good camera. The camera isn't the best in all situations, but it has a terrific macro and the low light shots are unparalleled by any other phone on the market. It's been a challenge getting used to the way Sense does thing versus stock Android, but I do love the flat, high quality design -- not to mention the 1080p screen. But I can't help feeling like I was more impressed with the Nexus screen for some reason. Sure, the One has the highest pixel density of any phone on the market, but for some reason it just doesn't wow me. The Nexus screen really blew me away (after my SGS3) after realizing how great the viewing angles were, as well as color accuracy. The image seems to sit on top of the glass, almost. The One doesn't quite have that quality. Blacks could be a little deeper, but it is an LCD and I realize that it's never going to be perfect like AMOLED.
Seriously though... when I used to see my Nexus sitting on a table I'd say damn, that's a sexy phone. Now when I see my One sitting on a table, I say to myself, damn this phone is gorgeous. I really wanted the black but after getting the Developer edition (only in white/silver) I can really appreciate the color scheme. It is just beautiful.