Staying with Günter was a pleasure. The room I stayed in was bigger than expected, cozy and full of light. Everything you need is provided. Keep in mind that you'll have to get to the 4th floor (there's no elevator) which might be an issue for people not mobile enough.
It's very silent considering you're in the middle of Berlin but you can still ger around very easily (subway is a 5 min walk).
Bars, coffee shops,.... are located near the appartement and easy to reach.
Günter is very easy to talk to and friendly.
I will consider booking again and can only recommend staying there!