Yesterday I saw an accesory for the spark and Mavic controller that slips on the stock antenna and supposedly increases the signal strength.  Is there a real advantage and noticeable improvement by using this device or is this a gimmick?

I would not recommend you use the third party antenna as it is not tested and put your spark under risk.

I think he means those plastic copper coated curved accessories that slide down your antenna to direct the signal back at them. I have one for my phantom and it does give me a bit of extra range but I have it more for better image transmission. It seems to do the trick.