TL;DR: Xavier's place is a great option for staying in the city, and a great alternative to staying at more expensive places near La Rambla, especially if you're mainly in Barcelona to eat.
Location: The apartment is a few blocks from Plaza d'Espanya, which is the 2nd biggest transportation hub in the city. If you like getting around by bus and train -- and in Barcelona, I highly recommend it since the transit system is really good -- then you can get really anywhere you want to in the city in 1/2 hour or less starting from there. There's also a cab stand for you bus-haters, or just for odd locations.
Maybe more importantly, the apartment is just down the street from Bodega d'Amposta, one of the best tapas bars in the city, and is about 1km over from the Poble Sec area that contains many of the city's best restaurants. There's also a bakery, greengrocer, fish market, and a xarcuteria within 4 blocks. The kitchenette is big enough to prepare meals, although you might want to suppliment some of the equipment. If you're in Barcelona to eat and drink, it's a great location.
The apartment is also a few blocks from Montjuic and the Magic Fountain. This means you can easily catch the display on Friday and Saturday nights (and you should!), and that going to the National Museum, the Miro museum or other Montjuic sights is just a hike uphill. If you're the type who likes a hike or run in the mornings, it's also great for you (I don't).
Amenities: the shower was great, something we tested heavily because Barcelona is muggy and we ended up showering twice a day. The bed is very firm, which is great if you like firm beds (I do). There is a washing machine, but drying is on a rack on the patio, so check the weather forecast before assuming that you'll rely on washing your clothes there. There's also plenty of space to hang out, eat, drink cava, or otherwise relax if you've already spent too much time walking around the city.
Do note that Carrer d'Amposta is a working-class neighborhood, which means that it's dead quiet at night, but gets noisy by 8am (except on Sunday). So if you plan to stay out until 2 and sleep in, you might want some earplugs.
Overall, a terrific place to stay in Barcelona and way more relaxing than a hotel. Enjoy your stay!