Must stay! Two of us stayed for five nights and loved the space. It has a great view from the balcony and a very comfortable bed. Poble Sec is an amazing neighborhood with lots of food and bars within less than five minutes walking distance that aren’t touristy. Quieter neighborhood for a densely packed city and it’s only a 15 minute metro ride to many of the major sights (Sagrada Familia, La Rambla, Barceloneta, etc).
One thing to be aware of is that that flat is on the sixth story (fourth floor) and has lots of stairs; if your mobility is limited this might not be a good fit.
Juan Carlos is also an amazing host who made all aspects of the process so smooth and he has lots of recommendations. Really can’t say enough good things about this place, just book it!