I have only joined one airdrop: the DeepOnion airdrop. It's a very rewarding AirDrop, but was pretty difficult to join in the beginning. There are only 882 active AirDroppers currently. A couple of months ago it was hovering around 500 for a while.
I think you should only join AirDrops of coins you are planning to buy anyway. Usually it requires some work before being able to join, so coins without a future seem a bit useless to me.