I rode the Renfro Valley DS last year and it was a real hoot, though definitelynot for beginners. It's set up for "B" enduro riders or above, especially if it's wet. Well organized (the same folks who put on the GNCC series). Saturday's ride was a genuine challenge (120 miles) while Sunday's was just about washed out due to heavy overnite rains.
Lovely trails, lots of varying terrain, and some pretty slick hills which caused just about everyone I talked to to eat it at least once (yes, I did too, on a slick, root-crossed downhill).
I would recommend riding with a buddy since hazards are present, though all turns were clearly marked and numbered. Almost don't need a roll chart, as long as some local doesn't screw around with the arrows.
Everyone should try it at least once - it's worth the effort.