The Cytus games are some of my favorite mobile games, if not some of my favorite rhythm games in general. The simplicity of the "slap the shit out of your phone" gameplay combined with the game's aesthetics and presentation keep me coming back for more.
While I'd recommend starting with the first one, this is a great sequel. I'm not a fan at all of how the paid DLC works, but it also has a really substantial amount of free content to enjoy.