It seems to be generally assumed (on SO at least) that there should always be unit tests, and they should be kept up to date. But I suspect the programmers making these assertions work on different kinds of projects from me - they work on high quality, long-lasting, shipped software where bugs are A Really Bad Thing.

If that's the end of the spectrum where unit testing is most valuable, then what's the other end? Where are unit tests least valuable? What situations would you not bother? Where would the effort of maintaining tests not be worth the cost?