I also upgraded to Premium just to have the playlist feature. A bit disappointed it's not available as it was the primary reason for upgrade. On the app, in the Alphabetical and Recent Downloads tabs it really needs to have an Add to/Remove From Playlist icon on each video item. Then another tab at the top called Playlist where you can access, manage and play your playlist.
It would also be good to have the add/remove from playlist icon on the media controller page as well as a quick way to get into the playlist from there.
Hope at least something is coming soon here!