I think they're sort of neat, a nice souvenier if nothing else... a nice way to give somone a pat on the back for a job well done... I've got a sack of them around somewhere. Most proud of my HOC and Chimo coins from CFS Alert.
Then again, a couple of years ago, we had a cpl who was a bit of a a**-hat, he had some sort of challenge coin, and was going around showing it to new privates, then demanding they buy him drinks because they didn't have a coin to reply with... I smacked him in the back of the head and told him to f-off.
I realise "smack to the back of the head" is no longer a sanctioned or authorized leadership action, but it seemed appropriate and suitable at the time. Of course the coin wasn't the root of the problem.