Quickly becomes impossible to progress without buying boosts and extra lives. I would definitely pay a one-time fee for this game because it's a lot of fun, but I refuse to keep paying for a game forever. Boooooo! If you ever decide to charge a flat rate like other premium games, I'll be happy to give you my money. Until then, I'll just uninstall and start over when I get to levels I can't beat anymore. Right now that's level 163.