Alex's & Natalia's apartment was great for a pair to stay in. The location was great, you have everything you need in close proximity (metro, tram-station, main train station, restaurants and grocery stores). We walked everywhere we wanted to get in Prague as the apartment was in the center of old town. It's also in a side alley, so you could say that the apartment was very calm and relatively quiet.
The only thing was that the apartment building is a historical building, with old windows and high ceiling and it was a little too cold inside for a winter stay (we visited on the end of january).