We have stayed two weeks at Alex's place. We both loved it very much.
The flat has anything you really need and especially the kitchen is equipped better than a lot of households. The WiFi has its high and lows, but it served our needs.
The location is amazing. The connection from the airport Tegel is really fast, you will take max 40 minutes to get there. A five minute walk from the flat there's the underground station Wittenbergplatz where you can take the U1, U2 and U3 and get everywhere without problems. Not much farther away there's the Kurfürstendamm and Zoologischer Garten. For the price it has the flat is an amazing deal, flats in far worse positions in Berlin can even cost twice as much.
There has been trouble with the neighbors once, so just remember not too make too much noise on the balcony.
Otherwise it was a pleasant stay. Alex´s flat is definitely recommendable.