We stayed on the fourth floor. The light in the stairway was not working so it was a bit difficult to navigate up the stairs but this was fixed while we were there. Make sure you bring luggage which you can carry upstairs as there is no lift.
The apartment itself was very clean with all the facilities you would require including free wifi. It was also a good size for Paris standards. The shower was a bit tempremental but nothing too bad. We were warned that if the hot water ran out that we would have to wait until the next day for more hot water. However we never experienced this and there were 4 adults staying there.
The bathroom is only accessible through the main bedroom which is slightly inconvenient, however it was very reasonable for St Germain so we were happy to overlook this. 2 of us slept on the sleeper couch which was very comfortable.
The apartment is located very close to a metro which means it's very easy to get to. I can definitely recommend the apartment if you are looking for reasonable, clean, comfortable accommodation in St Germain (a difficult thing to find).