The folks at BikeIV (Bike Illinois Valley) have done a great job building these trails. There is some smooth singletrack and some amazing elevation changes and beautiful views of the river. Some nice roots and rocks throughout the system.
Enter Mathiessen State Park at the southern entrance off of route 178. It will be the Vermillion River Area entrance. Look for a sign that says Horse and bike trails. It should be the first or second left. There will be an outhouse there. Park and find the trail head at the woods.
http://www.singletracks.com/item.php?c=1&i=4450