This is very pretty, and it's a very clever concept. However, I believe it's missing the key features of a dedicated playlist, repeat, and shuffle-- Ah, found it.
Kay! Well, I've got mine set to a 50% opacity, opaque over hover. It doesn't go opaque unless I hover over the playlist (main window hover doesn't do anything). Also, the progress ring doesn't fill as the song plays. Lastly, the playlist doesn't change color (besides the text) with the main window's setting.
This one's also very nice. But for the none-updating volume and seeker "bars". They don't update according to level and place unless you manipulate them using the mouse. I also noticed this behavior in the G7 skin.