Our stay at Jans flat was amazing the flat and room are exactly as in the pictures and Jan was great; showing us around and letting us know what was nearby.
The underground is only 1 minutes walk from the apartment and it takes about 10 minutes to get to the main tourist district of the city. It's also about an hour on the S9 train from the cheap airline airport so that's easy too.
I had a great time, Berlin it's probably my favourite city and I've been to a few!