I have owned the m3k for a week now - so I am posting some general impressions.
I purchased this as a dap for daily commuting and it's excellent for that purpose. UI is good, intuitive, simple to work out and no discernible lag. Easy to operate with one hand. I have noticed a problem with fast forward within a track - the player often stalls and won't resume playback when ff-ing - and I have to re-select the track to resume playback. Otherwise I haven't noticed any UI issues. I am using a 128 GB San Disc card, completely full, with no problem (haven't tried a 256). Build quality is typical FiiO - i.e. very good, and the small size is great for carrying around in a pocket.
Sonically the m3k significantly exceeds my expectations for a cheap dap. My other budget dap is a Shanling M0, which is $30 dearer but the m3k beats it in sound quality - not by a lot but there is noticeable superiority in some areas. My listening is mainly with Shure 535s and also Philips L2BO cans.
Claimed battery life is 21 hours. Can neither confirm nor deny - I have only gone 8 hours playback between recharges. At that point the battery indicator registers half full (for what that's worth). I think this unit could handle a long-haul flight no problem.
Background is quite black - can't hear any hiss at all with the 535s. (I don't own anything super-sensitive like andro's, but this isn't a dap for totl iems anyway).
Bass is well controlled with good attacks and decays. Layering and detail are surprisingly good. My main test track for that is Night Train by Christian McBride - a solo bass track with plenty of harmonic resonances and overtones and finger-slaps on the neck and bout of the bass that provide simultaneous contrasting woody notes. It's all there with the m3k - and the tonality is quite pleasing.
Mids are good, somewhat forward - certainly vocals sound forward in the mix. This has been tuned for everyday listening so the sound is relatively smooth and musical - but the level of detail would be considered acceptable in a $300 - 400 dap and is a pleasant surprise for a $70 unit.
Treble is less impressive than the bass / mids - noticeable roll off in the upper treble and I find there can be some glare or harshness with some higher pitched female vocals. It's not bad but if your musical tastes are treble-centric then the m3k is probably not the best choice.
Soundstage has decent width but no depth. There is no sense of any sounds being in front of you or behind you, but there is some sense of space left and right.
Overall, I think the m3k sets a new benchmark for sub $100 daps. Sound quality is clearly better than my iphone and is a step up from the X1 first gen which I owned. I can't say how it compares with the current X1 and X3 soundwise, but it wins on battery life and form factor. If you're looking for a budget dap and want maximum bang for not much buck, the m3k is probably the best currently available.