The Nena Apartments are an excellent option for exploring Kreuzberg and the surrounding area; very well located and the place is clean and well-equipped. It is run more like a hotel, with the staff on a reception desk during daytime hours. The rooms are hotel-like with a small but decent kitchen for cooking small meals.
Upon arrival, there was an issue with our bed - the slats holding the mattress were broken. We had a difficult time getting the staff at reception to accept the responsibility for this and get us into a new room, which go the trip off to a bed start. Their attitude was not very helpful; eventually, though, we were transferred to a different room and the mattress was fine. They eventually offered us compensation by giving us a late check in on out final at, at no charge.
Were it not for that initially poor customer service experience, I would have no reservations at all in recommending the apartments. They were an excellent base for exploring Berlin and everything was tidy and pleasant.