We stayed in Carmen's place for 5 days. The position is both nice and strategic: it's ten minutes walking from Plaza de Espana, Montijuc, Metro and Bus statiosn so basically you will be able to reach easily every part of the city. Moreover it's 20 minutes walking from the main train station (Barcelona Sants) and it's really handy to reach the airport El Prat because the Aerobus stops just in Plaza de Espana.
Just next to the apartment there is a grocery store and a guy who sell fruits and vegetables so that you will be able to cook whatever at home if you want to; if you don't there are plenty of little restaurants and fast foods in the neighborhood.
Carmen is very nice and has a lot of tips and suggestions to make your holiday even better. She is also very unobtrusive and you have your personal bathroom so you have 100% privacy.
I would suggest this place to everyone/every couple who want to visit Barcelona.