I have been there.
It is a state park. There is no fee that I am aware of -- at least there was no gate or anything.
The trails that I was on are fairly mild. One very steep hill, lots of mud near the lake if there has been rain. I was handicapped by brake problems so I kept to flat ground. The group I was with included a built jeep a newer stock jeep and me in a Pinzgauer 710M. There were a variety of trucks already there including a SPOAed Samurai and a few full-sized SPOAed trucks of the beater variety.
Naturally, tread lightly, pack in pack out, etc.
have fun!