DeltaT - I don't think it is FINANCIALLY worth doing. Unless it is as your are progressing to teach the IR, it'll take you a long long time to make back the money it costs you.
If you are currently instructing your school may sponsor you through it (mine did at the time).
If you arent looking to do it to make money / cover the cost by extra instructing, then yes it is a great course, and fun instructing. Very satisfying to see a ndb hold work, and makes the IR easier if you havent already done it.