First things first: yes, the apartment is on the 6th floor of a building with no elevator...
...but the walk up is so very much worth it! This is a beautiful loft space with two very large bedrooms, where our family felt completely comfortable and at home. The kitchen is among the best-appointed that we have ever seen in an airbnb, the balcony is a wonderful space to relax in the evening, and the presence of an air conditioner was so immensely appreciated during our August visit (and nearly unheard-of in Germany).
We booked our stay at the last minute (since we were having a very bad experience at another airbnb we had booked locally), and Babett and her team were wonderful and accommodating hosts from the very beginning, from arranging for an after-hours check in, to answering any questions we had throughout our stay (even in the evening).
Very highly recommended for your stay in Berlin!