We stayed there for 4 nights. The appt location is SUPERB, right in the center of everything. You can reach most of the attractions within 15 mins walk, plus the tram station and subway station are not far away.
We had a very pleasant overall experience.
But please note, that the appt is extremely small (great for 1 solo traveler, max 2, but you'll have to make room for each other) and has a couch (not a bed, as it might seem from the photos). And the couch is definitely not the best one.
Overall a great choice, if you want to discover Prague on foot for a couple of days!