OK, stable. It's been off, on, resuming, switching shuffle on/off, switching from individual tracks to playlists, etc. No problems at all. One problem I often experienced was Rockbox becoming completely unresponsive while the music kept playing. Nothing would get it to respond (no pause, no shutdown, no getting the screen to come back), only a Menu + Select reboot cured it. It was intermittent and hard to reproduce, but it hasn't happened in three days now. Not saying that's gone for good, but I'm hopeful.
It was more an exercise in, Will the latest UBI build work? The answer to that question is Yes.