Had a very comfortable and enjoyable stay at Martin's place!
I decided to stay there quite spontaneously and got a response within an hour. Check-in went smoothly, the apartment is very central. Especially the private bathroom was a blessing after a long day of walking around the city (great water pressure :D)! The kitchen is equipped for some uncomplicated cooking.
Everything was clean and tidy. By some mistake the towel in my room had not been changed and wasn't fresh, but I wrote Martin a message and got a new one quite quickly. Just a slight warning: the bed is super comfortable and wide, but not exactly suitable for tall people. I am guessing it's 180cm instead of 200cm in length, but I comfortable slept diagonally ;)
Would definitely stay there again when I get back to Barcelona!