I'm finally using Fit. I've been interested in it for a long while but I've never had a side project where I really though it would be useful, until lately.
One of my new years resolutions is to learn Latin. And, as usual, while learning that I began thinking about ways to apply it to programming such as an Android app to help someone learn and review Latin. Latin is like everything else invented by people: it's got complicated rules with many exceptions and special cases; perfect fit for TTD and Fit in particular.
I don't have tests around the android specific stuff like moving between actions and handling the gui and persistence, just around conjugating, declining, and translating Latin. Although it's been very helpful it has slowed large scaled refactoring. As I learn more about Latin I have to make major changes to how words and phrases are implemented and then I have to rewrite all the tests so they pass.
It feels helpful but I'll have to wait and see if maintaining all these tests really is worth the extra time and effort.