My family of 5 stayed at Frederic's apartment for two weeks. Communication with him and Mabel (from their office) was smooth, with always prompt replies to our queries. They provided us sufficient information prior to our arrival.
About the place: fits well to the needs of family of 5 with 3 kids. Apartment is very spacious,well lit all over which I appreciated well, big windows by dining room, cabinets in all bedrooms, enough pillows for all, kitchen equipped with utensils and appliance, high pressure water (also important!). Amenities have been well thought of to make the stay comfortable. Only downside is small shower areas in bathrooms which I think is the norm there.
Location: Area is safe, with groceries & some restaurants nearby. You just need to get to know the neighborhood and you will find what you need. It is definitely not as exciting or touristy as staying near Ramblas or Barrio Gottic, but I prefered safety over popularity when selecting a place. Metro Sant Pau dos de Maig is about 5 min walk and is on the same line as Sants station- helpful when you are traveling in other places by Renfe.
Frederic and Mabel also helped us arrange a van for our early morning departure going to the airport. Frederic is a great host!