We had a great time at Mar's appartment. It is located close to Catalunya Square and you can reach a lot of sights on foot. Also it is close to a bus stop and Metro line, so it is well connected to the airport.
The appartment is equipped with all the necessary utensils to cook food and has a good size for up to four people. Also it was very clean so that we felt very comfortable.
Mar communicated very well. He always responded directly with helpful suggestions and answers to our questions. The check-in worked without a problem as well.
The only issue we had was the street outside: It probably can't be helped in a big city like Barcelona, but it was quite noisy at night. So pack ear plugs to enjoy a good night's sleep.