Derek's flat is the best choice we could have made. My family stayed at Derek's for 12 nights (we spent Christmas and NYE there) and we felt at home. The flat is indeed immaculate and extremely comfortable. It is spotless, warm in the winter and equipped with quite new utelsils and furniture. By looking at the pictures, I thought the flat would be quite small (which is normally the case in cities like Edinburgh). I was wrong. The flat suits 4 people very well. It provides a two separate bedroom experience and a fair space in the living room. If you look around the flat you can see that Derek thought about all the details to make the guests comfortable and able to navigate Edinburgh. There are several travel guides in the flat with maps, a guide compilled by Derek with tips about restaurants (including the menus!) and the neighbourhood, and even a compillation of instructions of all the electronics in the house.
Derek himself is a great host. Communication with him was easy and quick. He is very considerate and always offers help. He offered to change the bed linen in the middle of our stay and also gave us some delicious treats for the Christmas/NYE, which was super nice and way beyond what is expected.
I would definitely recommend Derek's flat to anyone travelling to Edinburgh.