We stayed for two days in Washington at Adam & Jen and we had a great time. Because we arrived late they prepared everything that we can set up by our own. And it worked perfectly.
The apartment is very clean and nicely done up and the neighbourhood is peaceful. The Union Station and the National Mall are within walking distance so You Need no Car within Washington. Bike rental Station is nearby.
All in all: I can recommend Adam & Jen completely and would definitally book it again.