Marcel's place is bright, clean, and tucked away on a nice quiet street in Kreuzberg. The decor is funky and cool. The apartment is located on the top floor (the 6th, I believe) of the building, so if stairs are an issue for you, I would not advise that you stay here as there is no elevator. I personally love stairs since I find it great built-in exercise! Being on the top floor means there are several skylights (in addition to the numerous windows) that let in beautiful natural light.
One of the greatest features that makes this truly a gem is having access to the M41 bus that picks up on Urbanstraße, literally steps outside the building! You can ride the M41 to Potsdamer Platz (it goes past there too), which is within an easy walk to Brandenburger Tor. We rode the bus every night to eat dinner and every morning to eat breakfast. Did I mention Kreuzberg has amazing restaurants and bakeries! We used our noses to guide us, "That smells delicious! Let's eat here!"
This is a great place to stay in a perfect location!