The apartment is as small as you can imagine from the previous reviews. But: it's very suitable and functional for one, or even two for a short stay, person. The location is very good and peaceful. Very good public transportation, metro station very close (3 minutes walk).
If you like cooking your own meals the kitchen has everything you need, including an espresso machine. The nearby shops, especially the Chamberi Mercado (market) with fresh fish, fruits, vegetables and local cheese, is worth of visiting.
The only (slight) problem was to get the keys as there wasn't any written instructions. I'm sure Daniel'll improve on this!