I stayed at Peter's place about two months, and everything was great. Peter answered all my messages quickly, and was always available with all kinds of questions. Most of the time I was staying, he was away on a trip, but when he was at home, he always had time to sit down and have a nice conversation, so I really felt welcome.
The room was big, and quiet (rare thing in Berlin), and clean, like the whole apartment, and I had total privacy all the time. Excellent place to stay when on a long-term work related trip.
Only minus I can think of is the fact that it is on the fourth floor, and there is no elevator, but then again, little exercise never hurt anyone!
All in all I had a lovely stay at Peter's place, and I can definitely recommend it!