A few years ago this trail was closed, purchased by the City of Roswell, fixed up by RAMBO (Rowell-Alpharetta Mountain Biking Organization), and reopened. There is a 1.7 mile well-signed "Beginners Loop" from which all other trails connect.
Most of the trails are pretty easy, like Yellow River and Blankets Creek, but there were a few interesting spots on the smaller side trails.
There was a cool section of moguls, a few pipes, some more reasonable jumps, several creek crossings (some sandy some rocky), and a few good hill climbs too. All of these more challenging things were completely optional. There are loops upon loops so you can add in or take out just about any section of the trail. Even though there are a lot of loops it's bordered by GA 400 on one side, there's a creek in the middle, and subdivisions (and a swamp) on another side so it's nearly impossible to get lost.
How to get there:
The gravel parking lot is located just east of 400 on the left side of Old Alabama. To get there take Holcomb Bridge going East to Old Alabama, then go North on Old Alabama, and turn left into the gravel parking lot just past the entrance to Belcourt Apartments on the left.
The NAD27 CONUS GPS point for the parking area is 34 01 26.4 / 84 18 44.4.