I've been using one of these and it seems to be working fine so far. The crystals turn from blue to purple to pink as they absorbe moisture and then you plug the unit into an outlet to dryout. In the winter I needed to be "recharged" about once every two months but since it's been warm it needs to be dried out about every three weeks. My safe has ~22 sq. ft. of interior volume and sits in a laudry room which is sometimes humid.
A buddy has a golden rod but I wasn't too excited about drilling a hole in the safe and having something that could possibly cause a fire inside the safe. I understand that the limited amount of oxygen in the safe would probably chock out a possible fire but I figured why risk it if the crystals work.
I would also add that taking the guns out every so often and rubbing them down with a oil soaked rag is something that I like to do as an additional measure.