Alan and Kasia's apartment was a great base for our 3 day trip to Berlin. We were travelling with our 7 year old and she loved the big bunk bed. The location was fine. The tram stop is right outside and only two stops to the U bahn (or a brisk 10 minute walk) From here you can get to Alexanderplatz in 5 stops. There were a couple of small mini markets right around the corner and an excellent German restaurant (Budike) just 2 streets away. (the schnitzels were huge and the goulash amazing!)
The flat is basic but was fine for our needs. 2 bedrooms (at opposite ends of the flat) a good shower and a kitchen/diner. No lounge to chill out in, but we just used the bottom bunk as that was like a sofa. No oven but we didn't need one. Flat was warm but echoed so make sure you pay attention to the house rules and talk quietly before 8am/after 10pm.
Check in was fine. We didn't meet Alan or Kasia, although they kept in touch. A nice man called Victor met us at the flat and checked us in. We just had to email him once we were on the train from the airport.
If you are looking for something basic and quite central, this apartment would be a good base.