Firstly, Meli is a very kind person. She gave us many advice to come from Barajas Airport to this place.
Secondly, the flat is quite good for two to four people. We were two friends coming to Madrid to take photographs. Each one had its own room to sleep. The beds were quite comfortable, so was the entire flat. As this place is in an old building, there can be noise coming from other apartments, till 10 pm, but each one is used to living in a flat... So, it was not very disturbing.
Heating in this flat was quite effective and it was perfect !
If you're used to walking in the streets it's very easy to get 'Plaza Mayor' (15 minutes by foot, instead of '10' as announced) but 'Puerta de Toledo' subway station is not so far (10 minutes by foot)....
We had a pleasant stay and Meli is a good host.
In a few words, I highly recommend this place and this host.