My family and I had a great stay at Marleen's apartment. Location wise, it really was perfect- literally across the street from the U-Bahn, some good restaurants and a large supermarket at the end of the road.
The apartment itself was very big- if anything, larger than the photos suggested- and very warm. We struggled with the wifi a bit, but given we were there to see the city it didn't really matter.
I would definitely recommend this apartment as the ideal base from which to explore Berlin- thanks, Marleen! Ollie