I highly recommend Jannick's place. I only spent a weekend in Berlin, but Jannick was right on time with the keys, which made everything float smoothly.
He was well-prepared to show me everything I needed in his apartment and around the neighbourhood, wifi worked great. The flat is located on the third floor in a beautiful old building, and the flat is renovated nicely and has become a clean and bright home. Located on a quiet street, the balcony faces morning sun (it was fantastic for a long breakfast), but super close to the city centre, and Friedrichshain, and literarily only a hundred metres from the nearest U-bahn station.
I particularly fell in love with Jannick's balcony and his clean and comfy bathroom/bathtub - this is one of the listings that still feels like you're staying in a home and not a hotel-like copy. There was even some breakfast in the fridge for me for the first morning :)
I hope to come back soon, knowing that I can easily slip into this Berlin home again!