The apartment was located in a great area - tons of cafes and shops around. The building itself was well-maintained and clean. We liked the lift (one of our requests) that made carrying our many suitcases to the apartment much easier!
The apartment itself was very clean, with large rooms and all the amenities you can imagine - except air-conditioning, which is quite badly-needed in the summer in Berlin. Luckily there is a fan in each room, but you can still wake up hot and sweaty. My other concern were that the towels (though very clean!) were all threadbare.
We had a very pleasant stay overall, and our host was considerate, helpful, and always responded very promptly!