I would definitely recommend Carlos/Kmarta's place to backpackers looking for a decent place to crash in Barcelona.
The house was easy to find and is very close to a metro stop giving easy access to the entire city. There are a good couple of bars and eateries in the nearby vicinity as well if you need a quick bite before heading into the main city.
Carlos is a great host who went out of his way to make sure that me and my friend were comfortable. He was quick to say that we could treat the house just like it was ours.
The dog in the house is just funny; super friendly and was definitely a huge part of the experience of staying at Carlos' place.
Honestly can't think of any negatives. Would recommend without hesitation.