We stayed with 5 people at Samuele & Elenas place and had a wonderful stay.
The apartment is well equipped and conveniently situated. There are plenty of restaurants around the corner and a lot to discover in Trastevere. The bathroom is small, but beautiful. The bathtub is really inviting, but also a challenge if you're not taking a bath because there is no shower curtain. We really enjoyed our stay and would recommend everyone to stay here, if they are willing to have a true roman experience in the heart of Trastevere.
And yes you should be willing to take stairs since the stair well doesn't have an elevator and there are a lot of steps to walk up and down.