In response to a previous reviewer's comments and the trail description, this trail isn't hard to navigate. I believe the reviewer meant to leave comments on the DC section of the trail, which can be difficult to navigate in places and does lack helpful signage. However, the Montgomery County, MD, section of trail is well marked and easy to follow. Yes, there are several side paths offering access from surrounding neighborhoods, but if you go down one of them (more likely to accidentally happen if you're riding from north to south), you will soon realize your mistake and can easily retrace your steps.
As you approach Needwood Lake on the northern end, there is a massive sewerage redevelopment project, which amounts to a large section of trail replaced with old railroad ties. Not so good for road bikes but okay, if not bumpy, for hybrids and the like. The project is expected to take a while, so check the county's website for updates (http://www.montgomeryparks.org/PPSD/ParkTrails/trails_MAPS/rock_creek.shtm).
Per the description, the southern section of trail suffers from flooding and debris (mostly sand and mud) in places, as well as tree roots...this isn't a road biking trail. But most of the mid to northern section is quite debris-free in summer.
So enjoy the twisting, hilly ride through the woods. You won't be disappointed!