It was really nice being hosted by Ulf. It can seem like a scary neighbourhood at first for a non Berliner, but actually it is a really safe and well-located apartment. The neighbours are nice and the check-in is very easy to do. The apartment is very tidy and cool, with everything u need to stay for as long as you want. Be careful if u have heavy luggage since its on the 4th floor and it doesnt have a elevator, you'll have to be a little patient to get everything upstairs. Anyway, i highly recommend u staying at Ulf's.