Gun Safe Dehumidifier?

Just bought a heavy duty gun safe over the weekend and noticed there were 2 or 3 types of dehumidifiers on the shelves to help keep it dry in there. Do these work or just a gimmick? I know its important to keep moisture off of the guns but is it really a problem in these safes? It will be in MS if that makes any difference, so outside humidity can be heavy at times.

I don't mind buying one but I had never heard of it. Any body got any particular type they recommend? They had electric rechargeable, oven rechargeable, etc.