Avoid if there have been rains. In the dry one can do it in a normal car, but it would slow and rough. Ruts, rough, and rocky but I didn't see anything I couldn't do in a car if I slowed way down and choose one's path. No 4WD needed when dry, but a pickup or something with a good travel suspension makes it tolerable. It took me 70 minutes from the 310 to get there at a leisurely pace in my Tundra.
As others mentioned it's about 10 minutes to observe. Normally I would not recommend spending hours of driving for something cool but so short, but the drive is beautiful. One should think of the whole drive as part of the experience. Amazing vistas.
It's hard to see inside the cave, one needs a few minutes for ones eyes to adjust. A spotlight would be useful. Short walk from the parking area on a paved path with a few good stairs.
I drove up further as a friend has seen wild horses up there on top, but I didn't see any. Consider making it a loop by continuing driving that way which will come out at Lovell.