Only rode there once. Overall loop of dirt road with some single and double track off that. I only explored north side, Rabbit Run area was pretty fun. Not much for features or stunts if that's your thing. Lots of pines and needles on trails sapped your speed frequently.
From 495: Take exit 21B to Upton. Follow for about 5 minutes. Turn right onto Westboro Rd (Pratt Pond will be on your left). Follow for about a mile and a half or so and Upton SF will be on your right.
http://www.singletracks.com/item.php?c=1&i=4658