I just found out that you can now buy your ATV permits online and print them out prior to heading out on the trails. You can buy them here: http://www.marktwainpermits.com/
I've added ratings but honestly we only saw 10 miles of the trail system during an April, 2018 trip. The trails were open to 50" UTV's in January 2018 but the trails were not made UTV ready! We were the only UTV's at the trailhead. 90% dirt bikes, 10% quads and us! Didn't take long to find out why. These trails are way too tight for UTV's, even 50"! Was not enjoyable for my family. If the trails were widened it would be a LOT of fun in the hilly, rocky terrain.
Come prepared with a skid plate. This place is a meat grinder with rock everywhere. Great place to ride, low traffic. Best time to go is after it snows 4-6". Bring a buddy with a 4 wheel drive if you are riding a sport quad. Avoid during dry spells, dust is unbearable. Overall a great place with some very technical riding.