With my wife, Carla, just been recently at Bert's place and we just love it. Bert is an excellent host and will give you tips of really interesting sights of Berlin, besides to tell you the best ways to go to that points.
Also, his place is just like the pictures: an amazing old house located in the Kreuzberg area, close to everything and very well conected with the u-bahn, the s-bahn and bus.
Overall, if you could reservate in Bert's place, I definitly encourage to do it. You won't regret that.