As much as I've tried, I really can't get into "test first" development. I find that I make so many tiny changes when I am first working on a new project, and refactor so much, that the tests would really slow me down.

But once things get more solid for me (and, like you said, I know the code has some long term value), I'm a big fan of building a solid test suite before a full production launch.

I also think it is worth the time to use code coverage tools to find sections that are not being covered. Odds are your test suite only covers 60-80% of the total logic in your app.