We had a pleasant stay in Barcelona at Daniel's place. It is located near transports (metro and lots of buses), in a quieter area than the busy touristic spots, with a supermarket a couple doors down.
The place was rather clean and it was comfortable for our 4 nights there, we barely crossed path with the other tenants. Note that the building doesn't have thick walls, so we heard the neighbours often - bring your earplugs if you intend to go to bed before 11pm ;)