[PlunderBunny]

I wrote a game using a 'bottom up' design (i.e. IO and business logic first), and I wrote unit tests for the business logic as I went. With no UI, I effectively tested and stepped through my code with unit tests. I had the luxury of working by myself at my own pace.

I have a reasonably clean separation between the UI and the rest of the code, but I don't have any unit tests for the UI (I think - correct me if I'm wrong here - that would require integration tests rather than unit tests?) What I'm trying to say is that, if you don't do it this way around, and/or you have multiple programmers writing the game at once, and/or you _really_ optimise for performance, I can imagine that would make it much harder to write unit tests.