My parents and I stayed at Mark's place for 2 weeks when they're visiting DC. The apartment is exactly as shown in the pictures - very clean and has all the amenities we needed. The small office worked great for our stay - I used it as my bedroom and office and the futon was actually quite comfortable and can be definitely be used as a twin bed. Mark was very nice and responsive during our stay, made sure we had everything. The location is very convenient, plenty of restaurants and grocery stores on H street (within walking distance). My parents also enjoyed riding the free street car along H street, and to Union Station, where you can easily walk to Capital Hill, transfer to Metro, or take the free circulators to different places.
Overall we are very happy with this choice and would definitely recommend Mark's place for your DC stay.