I have a Sony NW-A35 player which is mostly working. I have two outstanding issues, for which you'll find dangling threads here, that I'm still struggling with:
1. Syncing files with special characters.
JRiver has an issue recognizing files on my device with special characters (i.e. artists such as Hüsker Dü, Béla Fleck, Moxy Früvous, Camille Saint-Saëns). So every time I sync, it deletes and re-downloads those tracks on my device. Of course one could be less fastidious and simply not use special characters in their library.
2. Playlist support.
This has been a very long struggle for me. I finally have playlists mostly working. They seem to need to be dumped, on their own (not in a separate folder) in the root MUSIC directory of the device/sd card, in the M3U (extended) format. The issue I'm still having is that JRiver writes a complete path to the M3U file, so every time I sync I have to open each changed playist in a text editor to change the path of every file (search and replace) so it reflects the presence of the playlist in the root MUSIC directory. JRiver seems to support relative paths for standard M3U playlists but not extended M3U.
That said, it's a joy to use a player that will handle every file format that I throw at it. It gets very tiresome always having to convert files for the Apple devices.
I should also note that this player, the NW-A35, is not running on Android. Prior to this, I had a NW-ZX2. One of the reasons I sold it (other than needing the money, it sounded great) was that it ran on an old version of Android and Sony seemingly had no intention of keeping the device updated. I've noticed no difference managing either the NW-A35 or the NW-ZX2 on a Mac with JRiver (same issues indicated above apply).