Extremely fun puzzle type game. Extremely saddened by the English server shutting down, however I'm still playing the Japanese version from time to time. There is a point where the difficulty spikes later on, and you most likely wouldn't be able to complete the stage without all Z rank puni, but there are plenty of hours of playtime before that. The side quests, collaboration events, and daily events do help this game keep you interested.
It is entirely possible to never throw down money towards this game as long as you are patient.