My wife and I went to Barcelona and stayed at the flat for 6 nights - we had an amazing time in the city and felt at home in the flat.
It is steps away from a great coffee/breakfast bar for the mornings, and many tapas restaurants and bars for the afternoons and evenings. We traveled throughout the city all week but always started and ended our days right by the flat.
For sightseeing, the flat is a block away from the entrance to a short hike leading to the Olympic swimming pool and an amazing view of the city. If you hike further up you will find a tram that will take you even higher up to an old defense castle, where you see both the city and the sea.
El Barri Gotic is about a 20 minute scenic walk away and is full of great bars, restaurants, plazas, and winding streets and alleys. Les Rambles is even closer, and we also walked to the Sagrada Familia. All of the walks are beautiful, and don't have many hills (which helps in the summer).
The flat is also only a few blocks away from the metro station - the beaches outside of Barcelona to the north and to the south are both about an hour away and are worth visiting.
In short, it's a great flat in a neighborhood full of character, is in walking distance of major attractions, and has easy access to public transportation.