Don't know of any apps like this, I've been thinking of a different kind of habit, or more like action tracking app.

You create different actions, for which you'll get different scores. Or there might be predefined actions to get you started. Scores can be negative. Every action adds or subtracts the current day's score by given amount, and the goal is to get the score for the day as high as possible. For example, you take the stairs and get 15 points. Or you smoke a cigarette, -50 points.

The app would have some features, like the obvious today screen to complete and add actions and show the day's score. In addition, you could maybe set a score goal for each day and check your score history, maybe with a nice graph.

My question is, should I go ahead and build this thing? Would there be users?