The apartment is quite good, but I'll start with the bad things:
- internet is a) very unreliable (there are hours when it doesn't work at all) and b) very slow, particularly in the bedrooms. If you need good free wifi this is not a place for you
- water heating system is not enough for larger groups of people - for our family of six we had to take superquick showers or take longer breaks between showers, since after 2-3 5-minute showers there was no warm water
- the apartment could really use some renovation; if you're looking for something special, this is not a place for you
Otherwise it's a very nice place to stay for larger groups. Very good location, just 2-minute walk from Metro station and in the middle of the city, so you can get fast to any interesting place in Berlin. The neighborhood is so so, but there are some nice cafees, restaurants and a grocery within 2-3-minute walk. The apartment is really spacious, bedrooms are large and the dining table can accomodate 10 people. It's also clean and has comfy sofas to relax + all the utilities you need. We had an overall good experience for a reasonable price.