This is a very fun loop, best ridden counterclockwise. The climb is stiff enough to get your heart rate enough, but mellow enough to enjoy. The woods on top are beautiful. Then, you get a long descent with waterbars every 50 yards--lots of oppurtunity for speed and air. Make a figure eight with the Rimrock Trail for more mileage, or you can even add on the Old Baldy loop for a true epic. Stop at the Park Service office in Spearfish and get the free maps. They are very helpful.
From Spearfish, head South on 14A to Savoy. Go west on FSR 222 for 4.7 miles to the trailhead. There's a campground there that makes a great base camp.
http://www.singletracks.com/item.php?c=1&i=2324