There are a couple courses around here with gravel tees. One is more like packed earth topped with gravel. The other is more like a shallow pit filled with pea gravel if that makes any sense. They both suck. The footing is very insecure, so people often tee off from the dirt beside the tee box. Defeats the whole purpose of installing the tee in the first place if you ask me.
I definitely prefer the rubber pads over gravel. As others have said, though, rubber pads are great until they get wet and muddy. Then they're slick as hell.
Honestly, I think concrete pads are the way to go. Unless they were covered in snow/ice, I've never felt insecure on concrete.