I love this trail. It is probably my favorite place to ride in N. VA. I like to ride cross-country with mixed terrain and this trail offers about every surface you can imagine. I have traveled most of its length over the course of various rides and have biked on pavement, gravel, dirt, rocks, and grass. I ride a mountain bike that has been geared towards mixed surfaces, so I enjoy going through the water at the fair-weather crossings and winding through some of the dirt sections. There are also some nice stretches of pavement, and even some occasional suburban street riding.
Some reviewers have said that this trail is not for hybrids. I would agree on some sections, especially on the northern end, but there are some really nice parts that would be a lot of fun on a hybrid. The section from Springfield, starting near the Fairfax County Pkwy, running north to Lake Accotink is mostly paved with beautiful scenery and only one fair-weather crossing (a stream crossing that you have to ride through the water, or get off your bike and walk over stepping stones). I would highly recommend this trail to anyone looking for something different than W&OD or Mt. Vernon.
Likes - A fun ride that alternates between a lot of different terrain. Beautiful stretches of wooded trails...sometimes you can't even tell you're in the DC Metro area. Very few of the type-A bikers that pass without warning, cut into the path of oncoming trail users, and shoot hostile looks at people not riding $2k bikes while fully clad in spandex (all common occurrences on the W&OD and Mt. Vernon trails).
Dislikes - Some sections get a bit rough, even for a mtn bike...they really should invest in a little maintenance along the northern stretch of Difficult Run. I don't mean paving dirt paths, but they could fill in some of the deeper holes and fix the washed out section near Great Falls. While the path is often marked with little CCT signs, they are inexplicably missing at a lot of spots where the path splits. If there is a junction, there should always be marker indicating the correct path. The CCT is more of a network of trails than a single continuous route. It is not always obvious which way to go at a fork. When riding sections that were new to me, I'd often refer to a map just to make sure I was on the right track.