Carlos and Reyes are super nice people! I studied abroad and stayed with a host family in college, and this experience felt similar. We stayed with them and their son (approximately 8 years old) in their apartment, and they were extremely welcoming and gave us tons of advice on places to see and places to eat. Reyes even helped us with our laundry and walked us to an internet cafe when we needed to print a document.
Other PROS: the location of the apartment was good, within walking distance of la ramble and multiple bus stops and metros. Although breakfast is not technically provided there was always fruit, crackers, coffee and tea that we were welcome to.
CONS: as Wisconsinites, we were unprepared for the Barcelona heat. This was our fault for not doing our research. There is no AC in the apartment, but a nice fan is provided. However we were still quite warm and had trouble sleeping a couple nights due to the temperature. There was a window in our room leading out into a courtyard, but it didnt provide much breeze. We had to close it early in the mornings due to some kind of construction outside. Also, Carlos and Reyes had another room with occupants and so we all had to share one small bathroom. This didnt turn out to be too big of an issue though as we generally slept in.
Overall, we had a good experience and I would recommend staying with Carlos and Reyes.