A great place to stay! The basics - cleanliness, comfortable beds, facilities, WiFi - were all there, but on top of that the apartment is tastefully and sometimes ingeniously decorated and any number of small touches, like having fresh milk, coffee, and a few other items already in the fridge just made settling in that much easier and enjoyable. We didn't do any cooking but the kitchen was well equipped and we could certainly have cooked our hearts out.
The immediate area around the building was pleasant and felt safe. There are bakeries quite close and you should try out the Italian restaurant on the corner.
You should be aware that you need to climb four stories by stairs (and the apartment has four-metre ceilings, so it's more like five stories for a newer building) and that there's no lift because it was built before that technology. Also, note that it's located in Neukölln and you will need to take public transport to get to the centre of town and other tourist destinations. However, public transport is good and the nearest station is about 10 minutes walk away.
I would thoroughly recommend this place to anyone.