I've been spending quality time with my AMP3 over the last week. So a few random observations:
FLAC doesn't seem to be well supported. After much back and forth, I still get clicks and pops (kinda like slightly used vinyl). I'm taking the advice of another member and just using MP3 files. As others have noted, the SQ is still superb.
The buttons do get much easier to use over time. I don't know whether it is the button breaking in or more just getting the feel for it. My first player had button issues, so I exchanged it. I feared that the button issues were more of a design flaw than a defective product. Not so.
The player abides by m3u playlists. I just create a folder and add the m3u file and then copy the files (using Media Monkey). I can prerandomized the lists and avoid the slow boot up and response times reported by others.
I have a 100 hours in and the sound has improved, although not dramatically. I didn't find the initial harshness reported by others (at least not on this player, my first player did sound very harsh at first).
On the interface, certainly, if you're a person you fiddles with the player a lot, this ones not for you. The interface is archaic. If you just load and play, then it is good. I did experience some freezes if I was trying to jump from song to song on the main screen. If you wait and allow the crawl to start, no freezes. Better yet, if you need to navigate quickly, go to the Music screen.
In reading back through the last pages and on other posts a couple of advantages get lost. Having a strong player in one package is a big advantage: no interconnects between amps, two batteries to charge, etc. I really like carrying this player around. The other is the long, long battery life. I've been playing this thing all weekend on one charge. I don't know how long but it is at least forty hours.
After some initial skepticism, I'm finding the best sound is with the RE0's. All sound good, but the RE0 is best. The stock buds are very good. I did some A/B with the stock buds and Yuins PK2 & 3. Couldn't hear any significant differences.
Hiss hasn't been an obvious problem for me, no matter the earphone.
Two things on volume. First, replay gain would be a nice addition. With a shuffle mix of songs I find I need to adjust volume frequently. Second, I'm using the 25 to 31 range for almost all songs. Older albums, like Pet Sounds, could use a greater volume, but not much more. I do like knowing I can crank it up without looking, knowing that I can't go too high.