OK, I gave this an aerobic difficulty of 8 because I hadn't done any biking in the two or three weeks before attempting this, so maybe someone in a little more shape would find this to be a six or seven. Not very technical as these are all car-free carriage roads built by John Rockefeller, Jr, but depending on the trails you take, you may have to face some 700-1000+ ft climbs on gravel roads. The views, especially in the fall with the color changes, are SPECTACULAR once you reach the peaks of the climbs. Coming down is a LOT more fun, though be careful in the gravel - very easy to slide out of control. Trails are also used by hikers and equestrians, so please follow IMBA trail rules. Overall, I rate this network a 10. Great to ride, great views, great history, great photo ops. Great place for a rigid, or even for singlespeeders after some pain. Also, great chances for some hiking - bikes are not allowed on any of the hiking trails, and there is no single track riding available on the Island (Acadia National Park). Also, some of the carriage roads are private, and are marked as such - please respect the markings and do not trespass.
All of the trail intersections are very clearly marked, though I would highly suggest purchasing a local map/guide book of the trails, which offer information such as points of interest, trail length, difficulty, etc. These also enable you to find your way back to your car, and you can create your own 'loops'. Again, there is some serious climbing to be done.
Roadies can also climb Cadillac Mountain in the park - the road up is open from one hour before sunrise until midnight, and due to the tourist traffic on the narrow roads, park rangers suggest that you attempt this in the earlier hours. Cadillac Mountain is 1532 feet high, and is the tallest mountain on the Atlantic coastline. Please note that not all of the park roads are open to cyclists.