My girlfriend and I stayed at Maria and Jorge's appartment for three nights and were welcomed kindly by their friend Paquita.
She explained us everything und showed us the accomodation which was clean and tidy.
You can easily reach all the major places in Madrid by foot.
Only thing to criticize is that it can be very loud at night because of the old window the appartment has and a bar in the neighbourhouse.
All in all we enjoyed our stay in Madrid and at Maria and Jorge's accomodation.