I'm leaving my evaluation lately because it was long to come back on earth after our amazing trip in Berlin !

More seriously, we were 6 friends in Ralph's flat during 5 days.
Although we didn't meet Ralph physically, he was a perfect host !
I mean Ralph was really sympathetic : at the beginning we should have been 8, then 7 and finally 6 in the flat. I modified the booking twice and he never complained. His neighbour was there to welcome us and gave us all needed information and several keys which it is interesting for a large group.

The flat is a little bit cold but is was not a problem for us during summer time as we spent most of the time outside.
Another detail: there is only one toilet, located in the bathroom. It requires organisation for a group of 6 to 8 persons :)
But this is really not a big issue.

Otherwise, the flat is really large enough for 6 to 8 persons. Everyone can sleep quietly. The kitchen is fully equiped even if we didn't use it as Berlin's restaurants are quite cheap. Plus, you got Curry 36 and Mustafas's Gemüse Kebap, two famous take away restaurants really closed (Mehringdamm station).
In the district (5minutes walk), you can also rent bikes and drop them in the courtyard of the flat at night.

Well, I hope you understand we had a great experience in Berlin and of course I recommend you to stay in Ralph's flat !!

Thanks again.