A very nice, clean, bright and spacious apartment a 10 minute walk from Hauptbahnhof.
The kitchen has all you could possibly need, coffee maker, kettle, microwave, gas cooker, ...
The living room is large with a comfortable sofa bed, the bedroom is pleasant although the bed is a bit too firm for my taste.
Felix is a great host, present without being intrusive and more than happy to give advice of sights to visit.
The neighbourhood is quiet, although during our stay there was a renovation ongoing renovation of an apartment in the building. Sadly the workers did not respect the agreed "start of the day" which was "absolutely not before 8am, preferably by 10am". They started drilling at 8.01am Thursday and Monday.
We were also quite surprised that we couldn't find a food shop any closer than Kaiser in the Hauptbahnhof.
However, I will definitely recommend Felix's flat to anyone travelling to Berlin!