If I'm writing Perl or something, I'll write unit tests just to verify the code runs in the basic cases.

I like Haskell because I can skip most of the unit tests. Integration tests are still good, and some unit tests like "confirm that test data are equal under serialization and then deserialization" help with development speed. But I can usually refactor vast swathes of code all I want without having to worry about breaking anything.

If you do write unit tests and your test passes on the first try, make sure you change the output a little bit to ensure it fails. It's more common than you'd think to accidentally not run a test.