I tried the new MBT from Union Station (southern terminus) northward, during July. The southern half is well marked EXCEPT for the section from the roadside trail (on sidewalk) to the off street dedicated trail at the Greyhound Bus Station. The trail signs lead to a set of stairs south side of the station, after passing under the railroad tracks. However, the bicycle RAMPS a block further north, at the northeast corner of the bus station. This should be corrected.
Traveling along the dedicated trail from there is okay, fairly level grade, but in the summer time it is very very hot. There is little to no shade, and being adjacent to an active railroad yard, there is a lot of heat build up along the tracks. Not that it is a hard ride, just that I would not recommend it on hot days, especially for children.
Once I got what should be about the halfway mark, the designated trail hits construction (07/12), then the markings become sparse and unclear. The landmark is Columbia University. Beyond CU the trail parallels an access road that serves a refuse transfer station. Again, not recommended on a hot day.
Once past the transfer station, the grade becomes very VERY challenging, and the trail signs become unclear. I took what the map showed as being the trail up to Fort Totten (a park and Metro stop). I believe this is the interim section, but the change in elevation in a short distance makes this a very challenging section. I am by no means a novice, but it was really tough.
The MBT is supposed to go to Silver Sring, MD, where it will connect to a number of fine trails. Unfortunately, I could not find any markers, and the one bike lane I found led to the south side of Tacoma and Tacoma Park, and another extreme grade (and no trail markers).
I will try the MBT from the Silver Spring terminus later this fall (when it is cooler!), and post a review, to see if it is posted any better. Once completed, it should make an excellent, if challenging, addition to the DC netwrok.