The UI is not amazing here; the OS is not Android, and so you don't get the intuitive ease and familiarity of an android UX. However, it's very fast and responsive, and it is reasonably intutive to navigate and build playlists. I used Sandisk cards and formatted them exFAT 512k. The library scan of the 2 full cards took about 10 minutes but now that it's done, it is no issue, since I turned auto scan off. I'm not displeased with the UX, but the UI is not amazing at this point.
Adding albums or tracks to playlists is very fast and easy to do, and it plays between tracks very fast, pretty much no lagginess at all. Where I did find it laggy and awkward was trying to use the tunein app that's built in. The UI screens (and in the player itself) are kind of hierarchical, and it's not always clear that when you back out, you might back right out. The tunein app actually is not very good. It's easy to inadvertently exit the app and have to restart it to get back down through the hierarchy to whatever station you were at. In this regard - apps - the UX (maybe because of the UI) is lacking.
I don't know quite what to make of the various upsampling options and filters. I'm not a fastidious audiophile guy who can rattle off elaborate impressions of what the sampling options do to the sound. The upsampling actually seems to make it harder for the unit to play some tracks. They definitely do change the sound, but I don't know if they are coloring it differently than it was intended to sound like. I don't know whether or not they affect the battery life, but I use it plugged in most of the time anyway.
As a player, it does what you want it to do, and plays all of the high resolution stuff, fast. I'm sure there are features that I haven't appreciated nuances of (or lack of), but it plays through lists and is easy to move around within album-length FLACs; as a player alone, it is doing the job and there have not been any real issues yet. I think if this thing holds up mechanically, it can stay with the high resolution digital audio product cycle curve for a good few years. But if phones evolve that can accept multiple large mSD card and run apps like Emby, Plex, or Roon, this type of dedicated unit would become redundant. That's maybe a topic for another discussion.
Hope someone finds this useful. The documentation and UX reports on this player are pretty slim at this point.