I was bitten by the TDD bug a few years back. I had been writing unit tests for awhile but the real value wasn't clicking. I watched a few Destroy All Software screencasts and then it really hit me.
To speed up the development of picoblogorg and to improve the quality of the code, I started TDDing the routes. Again, it reminded me how much better life is when it's surrounded by tests.
PicoLisp is a fanstastic language to write a small DSL in. The ability to pass unevaluated args is a dream.