On arrival (late, our flight landed after 9pm) Xavier and his wife were at the apartment. He was friendly, helpful and clear about everything.
The flat is lovely, like the photos but nicer in reality. Very considered decor and a nice, non-clinical feel. There were 2 of us but there is also a small second bedroom and the sofa looked like a pull out so room for more. The main bed is big (bigger than a UK double for sure) and firm. As the listing says, no aircon but overhead fans in the main bedroom and living room, which helped take the edge off (we stayed during an unusually - we were told - hot September, most days 30/31).
Shower is not very powerful but temperature control was good. We used the washing machine, stove and dishwasher without too much difficulty, though a little more explanation of these wouldn't go amiss. We weren't 100% sure what was fair game to consume (there was stuff in the fridge and cupboards, coffee, beer etc) so I hope we didnt take liberties. It might be worth asking.
The terrace is an obvious draw. We had breakfast and evening pre-dinner drinks out there most days. It faces an internal yard, sheltered from direct sun. As others have noted, there is a nursery/kindergarten 2 terraces along which runs from early morning till early afternoon. It is fairly noisy, woke my girlfriend up most mornings though we did have windows open at night.
The wifi worked fine, if not super-fast. I booked tickets, scoured maps without difficulty.
The area, Gracia, is a major plus. I lived in it for a year some time back and it is one of the jewels of Barcelona. You will certainly spend time in its bars, cafes, eateries and squares. Take a look at google images and read some web content if you need convincing. 2 metro stations on the green line within a 5/10 min stroll, line takes you straight to the center in 3/4 stops. Or walk down Passeig de Gracia for around 20/25 mins to the center, past 2 of Gaudi's most iconic buildings (Casa Mila and Casa Battlo) and lots of high-end shops. Also about 25 minutes walk across to Sagrada Familia.
All in all, great flat, great area, reliable and friendly host. If you are very sensitive to noise then the nursery may bother you. If you want to be right smack-bang in the center then this area won't work but reconsider that. Gracia is a little oasis with its own identity that should be visited anyway.
In a nutshell, highly recommended by me.