ID90 tickets tend to particularly beneficial for overseas travel. For example, I flew my family to Rome on my carrier. Then I used ID90 tickets on Air Malta and flew my family of four to Malta for about $120 total, round trip. I have friends who've flown to places such as Tahiti or Indonesia for similar cheap prices.
Be aware, however, that ID90 tickets usually must be purchased from your own airline's ticket counter or employee travel office -- not directly from the airline on which you intend to travel. So, you have to get them before you start your trip.
Every airline has certain agreements with certain other airlines around the world. Some airlines print a list of their reciprocal agreements. At other airlines, you may have do do some digging -- usually by asking an agent to check in the reservation system.
In any case, ID90 can be a great deal in certain situations.