We had a 3 day stay at Toni's apartment, and it was great! The location is very good, a lot to see and eat nearby, close to the train, but not too noisy.
The apartment is small, but that was no problem at all since we weren't in it except to sleep or rest.
Toni was very responsive and friendly, and check-in/check-out was easy to follow. If you're looking for a place to stay in Mitte, this is it!