Since version 3.8.2 Music Meal supports a sync service between all devices and instances. Currently it is the only functionality that requires you to register and log in. The sync service is currently limited to only your playlists and the songs in them. It will be expanded in the future to include your currently selected playlist, song, settings, etc, and eventually the ability to control one device from another, much like Spotify allows you to.
You can check out how the syncing is working in this short demo. It syncs between Chrome Extension, an Android phone and the web player opened in a different browser.