My boyfriend and I stayed for 9 nights at Christina and Stefan's place and it was so nice. They were both very friendly and nice and interesting to talk with. The room is really well equipped: extra reading lamps, private fridge, private WIFI router, big TV with so many cable channels, and a very cool amazon echo in the private bathroom.
The place is not very close yet convenient with public transportation to the city (10 mins walking to SBahn and then 20+ to Friedlichstr.). I personally like the location as it's a quiet neighbourhood and mentally you felt more relaxed than in the "hipsters areas".
Highly recommend!