I'm still only getting an occasional chance to use it (like it very much), but I have a couple of usability comments. Thank you for the power button (my request)
1. The android back button should take me back within the app - e.g. I'm looking at an album, back should take me back to the full library list. From now playing it'd dismiss the now playing and take me back to library. I'd suggest that when there is no obvious back action, it should pop up a "do you really want to quit" dialog, which back again would cancel. This would match many apps I'm used to at least. I keep hitting back and being thrown out of the app.
2. I'd like to see search being a filtering restriction applied to the library, so default library view is unrestricted, typing makes that list collapse to the matches. The current model where search gives a popup is a bit clunky, and then once you select one of the matches, as far as I can tell you can't get back to the search results (?). Anyway, the narrowing filter search model is pretty widespread and works nicely for me in desktop music players.
One other thing is I was hoping to add a currently playing track to a playlist, but didn't see an option - actually don't see a lot of playlist support. Just a use case to bear in mind.