I stayed in Richard's flat for a month. It was a very nice stay overall. As a lot of the reviews here say, the flat is very spacious. There's enough room for a whole family to stay comfortably.
It's situated in a lively area, with some of the best restaurants and bars in Berlin right downstairs. This does mean that there's some noise at night, but that's the trade-off for staying in an area like this. It's less than 5 minutes from Ostrkreuz station.
The internet is fast and stable when connected with an Ethernet cable. I got a consistent 70-90Mbps. The WiFi is less stable, but perfectly fine for casual use.
Richard and his partner were very friendly, and immediately fixed any issues I had.
Overall, a great place to stay that I'd recommend to anyone wanting to sample Berlin culture.