Don't really like rhythm games, but this one is exceptional. I love Cytus because of it's uniqueness, both gameplay and the story are very satisfying.
So far I bought every character (except Ilka) and some collab chars (Deemo duo, Kizuna Ai, Miku and Amiya). But what I like about this is that you don't need to buy anyone and anything to enjoy the main story. If you are interested in a particular character you can always buy them, as well as some song packs that got your attention.
P.S. don't know what happened after I uninstalled the game 5 or 6 months ago, but as I reinstalled it today it seems I lost all my progress. I have had to reinstall it for a few times long before that, and after syncing the progress always updated. It isn't a big problem tho since I can always play the songs again. The unfortunate thing is I MM'ed some difficult songs with pure luck. Hopefully I can do it again after a big pause :D