I'm lucky enough to have a Kobol Helios64, but unfortunately the small company that made these shut down. It's fine for the time being but I'm going to have to pay attention to the NAS market to be ready to replace it one day... my main goal is low power, so I'm not sure if it's worth it to go to a more commercial option like Synology or if I should be building something.
As appliance NAS tend to be, the actual SBC in the Helios64 is pretty slow so I minimize what I run on it. It does have Plex server, but most everything else runs on another ARM machine that mounts from it by SMB.
I still miss my old iRiver with Rockbox... definitely been thinking about getting a proper PMP again.