I visited the apartment with my girlfriend and 5 friends for 4 nights. We had a great time in Berlin and the apartment was a part of our great experience.
The apartment is basically an open space with separated kitchen and 2,5 bathrooms. Staying 7 people together I believe that having 2,5 bathrooms was the best asset of the apartment and one of the reasons I chose this. The open space also mean no privacy, not even for the separate bedroom as the door is more like a curtain rather than a solid closing door.
The location is really nice. It is not close to the city center but it's next to a subway station that takes you directly to the center, so no worries about that. Also it is next to the Stasi Museum which was a nice thing to visit before our midday leaving flight.
It's on the ground floor but we had no issues with noise or scene of danger (very quite and safe neighborhood).
Being picky though, there are some things we did not like which did not affected our experience and thus not reflected to the score rating:
1) The coffee machine was forgotten dirty
2) Some pillows need replacement
3) The towels were rather small
4) No first aid kit (or at least we did not find one)
Overall the apartment is highly recommended and to be honest we already have done so to friends who want to visit Berlin.
PS: A small tip for group travelers - Buy the day group ticket for public transport, it is well worth it! (20 euro for 5 ppl)