Lovely apartment but not for three people. The sofa in the guest room was extremely uncomfortable to sleep. We stayed there just one night, so it was ok but a longer stay for three people would not be recommendable. Despite the location it was very quite and peaceful.
Flexible check-in but unfortunately has to be performed in person. Since it was our vacation and we were driving out from Marbella - you are sort of under constant time stress if you are going to manage or not. It is not a private host but it is a company which rents out many apartments in Madrid, so employee of the company was waiting for us which was very kind since we came with one hour delay.
The bedding for the sofa was not prepared and you had to do by yourself which is fine but would be nicer from the host if it was already prepared. One towel for face was missing.
Altogether if you need one night stay in central and quite interesting neighborhood, then it is recommendable.