We stayed at Nikolai's for 5 nights recently.
Nikolai was helpful and attentive. He came home early from work to meet us when we first arrived, showed us around and returned to work. His flat is as it appears in the pictures. The bed was comfortable and super long. We had access to the kitchen - great to make EXPRESSO each morning, and Nikolai had fresh rolls, cheese, tomatoes, jam available for us. He has a washing machine and washing powder that we could use had we wanted to. He was sincere in wanting us to feel at home in his flat.
The flat is in a great location, with the tram just outside the door. It is also one stop from the S-bahn. (So yes with the window open, we did hear the street noise, this is drastically reduced with the window shut). It takes about 10 minutes on the tram (Tram 4) to get to Alexandra Platz and about 10-15 minutes on the tram to the Central Train Station. (Tram 10)
In the area, there are some restaurants and bars, a bakery right below the flat (we bought some tasty croissants and sandwiches there the day we left) and Prenzlauer Berg (trendy area with lots of restaurants) is about 3 tram (M10) stops away or a pleasant 20-30 minute walk (we took the tram there and walked back)
Nikolai was accomodating, quiet, (one or two nights we didn't even see him) and friendly, offering some suggestions about things to see/ do in Berlin.
We had a good time, glad we stayed at Nikolai's.