The location couldn't have been better: a really quiet street with leafy courtyard at the back, about a 2-minute walk from Spittelmarkt U-Bahnhof and 6 minutes from the bus stop. We were touched to find some mineral water and sweets for us upon arrival, as well as maps and catalogues about Berlin. The beds were so comfortable and as someone who finds it difficult to sleep if there is noise, the apartment was almost as quiet as my home in a country village. The pillows are lovely and squishy so if you like firm pillows to sleep on bear that in mind, but we slept just fine :) In the main bedroom there are two single mattresses pushed together on a king-size bed frame, and also the sofa-bed in the lounge, which we didn't need to use as there was ample room on the two singles and two single duvets.
Marc is a wonderful host: friendly, accommodating, and super-quick to reply (he even replied to me in German so I could practise, despite his English being excellent!)
My sister and I had an amazing time and would definitely love to come and stay again when we visit Berlin next -- which we will have to as there is just so much to do in this great city.