Overall, I would highly recommend this listing. It's a very convenient location to much of what you would need for both short term (e.g., touristy things, the beach, etc.) and long term stays (e.g., grocery stores, coffee shops, gym, etc.).
The apartment itself reflects accurately what the listing portrays. The bed is comfortable, the kitchen is great, and the bathroom is quite clean and nice. As several reviewers have noted, it's towards the back of the apartment and has only one window which overlooks an enclosed space. Having said that, most of our time at the apartment was spent during the evening, so this wasn't a huge issue.
The apartment is also well-air conditioned and keeps the apartment cool on hot Barcelona days. One note is that it has to be turned off by 1 A.M. each night, specifically for neighbor noise. While for the heat sensitive this may seem hard, my partner and I found that running it throughout the day helps to keep it cool throughout most of the night. Marta, too, provides a very powerful fan that does a decent job as well. I should also note that on a couple of occasions where we would be arriving home later than usual, a simple text to Marta was met with quick responses to leave it on past the 1 A.M. mark so as to arrive to a cool room.
For those staying longer term and possibly working remotely, something to keep in mind are the wifi issues city-wide. This is unrelated to this apartment, but there were instances were wifi was hard to come by and made work a bit difficult. Marta, though, sent continual updates on its progress. This communication by Marta meant a lot, and was evident in other areas of our experience.
One last note: we had an extra guest come stay with us for a week and Marta was more than accommodating (and even provided us with extra blankets and pillows!).
Would definitely recommend Marta's!