I've received my M3K today, and here's my hot take:
It is a really pretty object. I like the feel, the weight, and the sleek looks. Even the silicone case has a snug fit and makes it look pretty neat. The screen looks better than I had anticipated, considering the size. I am someone who really likes to see the album art while playing, and although they are obviously small, the screen is bright enough and has enough resolution to allow to read even fairly small album titles on the covers.
I am far from an expert (my previous DAP was a 5.5.Gen iPod classic (the famous Wolfsom one) and my desktop DAC is a FiiO E10K (so very far from endgame stuff). However, this little M3K has quite surprised me for how clear and dynamic the sound is (I'm playing various FLAC files through my M50s). Definitely a step up from my iPod, and frankly a good match for the E10K too.
I have two complaints that I really hope FiiO will address in the next firmware update (if you are reading guys, please take notes!):
1) while the EQ presets are actually fairly good (although they kill a few Dbs of volume), having manual EQ would be a MASSIVE addition;
2) I have over 300GB of music on it, just over 1900 albums. Scrolling through the list takes quite a while. It is a bit faster using the "skip track" buttons, but still a bit too slow for some occasions. To patch this and add some incremental form of scrolling speed (or even simply keeping the scroll bar as it is, but making the "skip track" scroll speed much faster) would be quite necessary to be honest.
These two caveats aside, for what I paid for it it is unbelievable value for money. I still can't say anything about battery life (something I care about a great deal), but all reviews are in agreement about it being really really generous, so I'm not too concerned.