A great flat! I took an express bus from the airport to Plaça Espanya (€5,90), and a cab from there (€4) which was suggested by the host, Martini (he asked if I wanted to be picked up at Plaça Espanya but I decided not to be pampered).
The flat is only minutes away from the subway station Placa de Sants (L1-red one/L5-blue one) on foot , so with Barcelona's great metro system, I was able to go everywhere I wanted to visit in the city so easily. There's a supermarket (Consum) only a minute away, and a cafe just around the corner (both literally) where you can have a morning cappuccino at a local price. Nearby shops offer fresh fruit. The area is nice and quiet.
The flat offers a terrace, where I used to shake off my jet-lag with beers in the sun right after my arrival. Got a nice morning tan everyday there.
As for the host Martini, he was amazing. He gave me detailed info from the get-go so I already felt welcome even before I arrived (what to say to a cab driver in Spanish helped a great deal). During my stay, he gave me all kinds of useful and accurate tips. He's not from Barcelona, and because of that, his info is really useful for a traveller, and he knows the city so well. Martini did go overboard to make sure that not only my stay at the flat was comfortable but also my time in Barcelona would be awesome. Thank you so very much, for everything ;) You are super!