We were 4 young people staying for 5 days. Overall we can say:

Pros
+Excellent location, close to the city center, the metro, and any kind of shops, full of nightlife;
+Well kept, new forniture, very clean overall, and well furnished with basics;
+Relatively large to be an aparment in the very center of Madrid, and with a quite useful and quite cozy wide terrace. And TWO bathrooms. Don't forget that;
+THE LOCATION. Seriously, in a few minutes walk you could reach anything.

Cons
-Just fine for 4 people, but it might feel clenched for 5 or 6, althought there are up to 6 sleeping accomodations, unless you're very intimate, it can become uncanny;
-Great nightlife also breeds some annoyances. One night (morning?) we were waken up by somebody repeatedly buzzing our bell and the neighbors'. At one point we heard some heavy footed spanish guy also living in the same building rushing downstairs in a not-so-joyful mood. I hope the buzzer is OK. And it seems that the day after we were somehow blamed for the episode.

In conclusion, I have to say, great stay! If we'll return to Madrid, we'll know where to go again.