Me and 4 friends stayed in Tobi's place for the whole summer, and it was great! There was plenty of space, and lots of couches for chilling. The location was perfect, near a train station and lots of supermarkets. On the trains, it was only a few stops away from the clubbing district, lovely parks and shopping areas.
Although Tobi doesn't live in Berlin, he rang a few times to see if everything was going okay and so on, which was helpful.
I would definitely recommend this as a place to stay when in Berlin.