I have ridden the Mammoth Classic both years. It put on by a great shop in a really neat little town about a hour north of Minneapolis/St. Paul. I have alway done the longer routes (70 and 100 mile options). Both will take you up into the "sand barrens" and will route you through a really amazing network of fire lanes.
My experience from
2013 and
2012.
If you are in the upper midwest, its worth the trip.
Joshua