It's not that big, but it's one of my favorite places to ride. There are a few awesome windy, flowy singletrack trails of decent length, some great climbs and descents, a fun pump track, and you can go after dark up until 8pm I believe. It's also not a bad place for a short hike and snowshoeing, which I was doing there last week. Oh, did I say it was small? Well, it's pretty much connected (very short ride through a residential neighborhood to get there) to another small town forest that has some really fun short and windy singletracks...from there, it's a short 1/4 mile ride to Billerica State Forest which has some more really fun, windy and flowy singletrack loops of decent length...and it's short and easy access from Russell Mill to Great Brook Farm State Park which I've read also has great singletrack MTB trails (I've only hiked there). Russell Mill might not be huge, but combined with the other trail networks I've mentioned, you could easily get an incredibly fun and challenging 15-20 mile, and probably longer, ride that didn't cover too much trail more than once. For all of those reasons, Russell Mill is easily 5 stars in my book.