My brother and I stayed in Alexander's apartment and we both really enjoyed it! The space was more than big enough for two of us, but I think any more than that and you'd begin to feel cramped.
The bed and sofa were comfortable to sleep on, the shower was warm, and the kitchen was big enough for our needs. When we were in Berlin it was very very cold, but the heating in the apartment was more than up to the task! The tv was equipped with Netflix and Spotify among other things, which was a nice surprise!
The only negative was the smell of the drains in the bathroom, which at times was quite strong. That said, it wouldn't put me off staying at Alexander's place again.
Overall, a really enjoyable trip.