We had a great time at Carlos and Frank's place. Communication was superb and they were very flexible regarding the check out time.
The apartment is very modern and was spotlessly clean when we arrived.
The area is ideal for exploring the rest of Madrid either by foot or starting from the Metro. The 24 hour supermarket just across from Lavapiés Metro is very handy.
As mentioned before the area is pretty hip and on two occasions we were approached by drug dealers. After refusing their offer they didn't bother us but people with children may wish to consider this.