TDD, for me, is great at the very start. I get the nice high-level bits done, they're clean, and they get things working perfectly from the start.

But then I want to start moving fast, trying new things, I don't know exactly how I want to go about implementing things. I end up passing on tests for a while, until I hit that next threshold - then can go back; make tests for what I wrote to catch up, and then repeat.