Elfa's place worked out great for my friend and me. Elfa was happy to answer any questions, and she left a list with the apartment rules which was helpful. The apartment is super close to the subway and only a short walk to La Sagrada Familia. I'd suggest taking the RENFE (train) from the airport and transferring at Passeig de Gracia to take the subway to Monumental to get to the apartment. Or you can take a bus. My friend and I bought 10 trip metrocards for our long weekend in Barcelona which is something else I'd recommend to get around the city easily.
If you're looking to stay in a clean, quiet apartment shared with a kind and positive host, then Elfa's place could be great for you. We kept to ourselves for the most part, but it was nice to know that Elfa was there to accommodate us.