Me and my boyfriend had a great stay in Madrid in Melvin'a apartment. The room is rather small but it is well equipped and if you don't intend on staying there the whole day it is absolutely no problem, we found it quite cozy actually. Location is amazing, close to everything and we didn't take public transport once. Everything was very well organised and we encountered no issues at all.
Only one thing - keep in mind that being so close to Grand Via the area could be a bit noisy at night (especially during the weekend) and also walls are rather thin so you could hear the neighbours if they aren't being considerate with their noise levels. That said, unless you are exceptionally sensitive it probably won't be a major issue. It certainly didn't keep us awake for long or anything like that.