Military unit coins, are there any mylotters here who have one, care to tell us?

No, they are not for payment of anything. I usually don't consider Wikipedia to be a good primary source but this article is pretty good at explaning, take a look. http://en.wikipedia.org/wiki/Challenge_coin

Besides using coins for challenging, they are also used as rewards or awards. They are used as a tool to build morale. In the context as they are used by the modern U.S. military, the tradition probably began amongst Special Forces units during the Vietnam War. The tradition spread through the Airborne community, and by the early 1980s also into the 75th Infantry "Rangers." As officers were reassigned as their careers progressed, they carried with them the tradition of awarding a unit coin for acts that were worthy of recognition, but yet lacked enough merit to submit the soldiers act for an official medal. Challenge Coins were not very common until the First Persian Gulf War of 1990-1991, and have steadily grown in popularity since. While most soldiers are happy to receive an award of any type, some feel that there are so many coins being given out now that their value has been cheapened. I think it is a good way to get a free drink if you are the challenger and only do so if they are for sure they don't have their coins because otherwise you'd be the one buying the drink

How cool! Thanks.