Personally, if anyone asks me if they should unit test, I'll play the fanatic and take a hard line. Mostly because the way to learn the usefulness of unit tests is to write them. You quickly learn what they're good at and what they are bad at.
Once you've written enough of them, you kind of seem to develop an instinct as to what unit tests are useful for.
Once you've written enough of them, you kind of seem to develop an instinct as to what unit tests are useful for.