It's ridiculously fun, but I feel like an extreme mode could be useful, since once you get enough upgrades, it gets a bit easy to beat a level. (Alternatively, I suppose I could just remove the upgrades. ;))
You might want to know that there are holes in this game, literally, you can sometimes exit the tunnels, you might want to work on that, but, discounting that, this is a seriously awesome game.
I think they've been there since day 1 to be honest :( what an idiot developer ;)