We had a wonderful stay at Carolin's.
The apartment is spotlessly clean with comfortable beds and all the amenities that you might need. Public transport is very close and you can get around Berlin with ease. Restaurants and bars are close by a few minutes walk away. I would recommend Fleury Cafe and Distrikt Coffee for excellent breakfast both close by in Mitte.
Armin met us on arrival and explained everything he is very kind and charming.
I would highly recommend staying at Carolin's.