The location is in Kreuzberg, close to public transit stations (bus, U-Bahn, S-Bahn), which was nice. It also has some shops and restaurants in the area which are nice, we especially liked the Turkish breakfast place right on the corner, they served some amazing scrambled eggs with some hot bagel.
The apartment itself was on the smaller side, but clean. You have to get a bit creative if you want to be able to put 2 suitcases somewhere and search for clothes at the same time, but we managed.
The checkin was also ok, don't know about the alternative checkin, if you don't get there in the timeframe when the reception is empty though. Checkout is super easy, we just dropped the keys in the dedicated mailbox inside the building, near the exit.