Longtime lurker here, figured it's time to post my J3 impressions. I've had mine for about three months, and typically pair it with a Fiio E11 and a variety of headphones/IEMs.
Things I love about it:
1) Plays mp3/flac/etc in same music player
2) Expandable (microsd)
3) Hardware buttons
4) Slim, pocketable form-factor
4) Playtime (64 hours)
5) Moddable UI (currently using Kizune's
Lynx, Sense and Leaf mods)
6) Wide variety of default and customisable EQ presets
7) Drag and drop interface
8) Build quality
Things I don't like:
1) UI by default is pretty clunky and unintuitive for first-timers
2) Micro-SD/power input "protector/guard" is annoying - I ripped it off
3) Charger cable is different to everything else I have
What would I change/improve upon?
1) Add a line out
2) Improve UI
3) Add (world) standard charger port (mini/micro-USB)
I've spent a lot of time looking for something better, and haven't found much that comes close. There are several devices that purportedly offer greater sound quality (HM-601/etc), greater outputs (S:Flo2), but usually fall short in terms of design, price or build quality. As I'm still struggling to find anything comparable, I'm honestly wondering if I should buy a second unit while they're available.
Overall rating: Excellent, must-buy at it's price point.