For me the primary use of unit testing is in the prototype stage. I write the tests instead of the application code to make sure everything is sane. I find lots of architectural/assumption problems this way.

After you've got real app code the tests serve more or less as regression tests to make sure I haven't broken anything major deep in the guts of my models and such.

In short, like some of the other comments here: Unit tests aren't for finding bugs, they're for helping you not write the bugs in the first place.