Casa Cosi is a well-located apartment with a lovely terrace. We enjoyed both starting and ending our days sitting outside. We were able to walk to most sites within 30-45 minutes max (many closer). The management was extremely responsive -- even helping me find a place to purchase a new electrical outlet adapter when I forgot mine!
That said, future guests should be aware that the second bedroom is extremely small. It's basically the bed plus two small tables on each side. There is no other space in the room to hang out or relax. Also, the bathroom in the master bath has a fully glass door so the toilet has no privacy whatsoever. We had a mosquito infestation while we were there -- we didn't have problems anywhere else in Barcelona so I hope that the management will be able to eradicate the issue in short order.
Otherwise, the location is excellent, the building is quiet and the amenities were very good.