I had wonderfull time during the month I stayed at Alex and Michael's apartment. It is exactly as they told at Airbnb site and it has everything you need to stay very well, furthermore, it is very well located. The U-Bahn station is close and can take you where you want to go in Berlin, it is easy and practical. Michael is very friendly and he is always ready to help with something you eventually need. I think the option to stay in Michael's apartment was a good idea.
Thanks for all.