We found the location to be ideal for our needs. It's close enough to the main sites to be easily walkable, there are lots of restaurants and bars very close and for anything further afield the metro is nearby.
The apartment itself was clean and had everything we needed. We particularly appreciated how well stocked the kitchen was with the small things - oil, salt, pepper etc. The coffee pods and bottled water were also nice touches, as was having Netflix on the big TV.
We love Madrid and would certainly stay in Gertjan and Johan's apartment when we come back.