It's a decent experience that has several drawbacks.
It's managed by a company (Stay U-Nique) which means they are obviously focused on having the highest possible turnover and profit. I've previously only stayed in apartments that were privately managed, which I think is much more preferable - it's just more personal and I've gotten to know some great and truly charming hosts over the years.
Private hosts usually deeply care for their places and put a lot of love into it, which was lacking completely from this apartment.
Yes, all bedrooms are noisy at night, but not to a degree where it kept us or our two small children (3 and 1) from sleeping. Generally, the apartment was great for the kids, no hazards and a lot of rooms and beds and couches for them to jump and play on.
Check-in was ok, but also pretty annoying that you have to define such a specific time frame beforehand, as it is obviously tough to judge how long you are going to need to get from the airport to the apartment. If you arrive late, you have to pay extra fees, and they definitively aren't shy to remind you about it - which I don't think any private renter would dare.
Also, our check-in agent told us there wouldn't be a baby bed or high chair because we didn't ask for them specifically in advance. Which obviously was disappointing, because they knew we were coming with two small children on an easyJet flight, so chances are we're not bringing all that stuff with us, right? The agent was helpful in providing their office number and promising that they would take care of it as soon as possible - which would have been on the next day, as we arrived around 8:50 pm on a Friday.
However, once we started exploring and checked all the closets, we actually did find a baby bed and a high chair, which was great. Just another thing that wouldn't happen with a private host. And it was curious, because our agent supposedly had been waiting there for us for about 30 minutes, you would have assumed she would have checked the closets themselves, instead of flat out claiming we're out of luck for the night. Just glad we checked for ourselves.
Another minor nuisance was how needy they beg you for your 5 star rating, I think I've gotten like 5 messages subtly nudging me towards giving them a premium rating - I just felt reminded of that one Black Mirror episode where every social interactions is rated all the time... yikes!
The upside here is that there is an experienced company always at hand. It seemed like they had taken previous critiques seriously. We found a bunch of stuff taken care of that was complained about in other reviews (missing lights for example - though the ceiling fan still just lays under the bed in the green room). Communications was friendly and they had a quick response time, even on the weekend.
We had a problem with the heating, which would only turn off completely or run at full blazing heat. They offered to send someone to look after it promptly, which we declined because we didn't want to deal with it and just worked our way around it.
Cleaning was adequate, though I suppose if you are super anal and inspected everything, you probably would have found stuff that bothered you, just because you can tell the place has seen a lot of mileage. Apartment could generally do with a fresh renovation some time soon.
There were several small issues (the fan under the bed and the corresponding holes in the ceiling, a tear in the fabric in one of the hallway lights, missing shelves that weren't replaced, permanent stains on several surfaces, etc.) - all stuff that would have most likely been fixed and taken care of by a caring private host, whereas this just felt like noone bothered because it wasn't necessary and would actually cost money.
What I personally loved about the place was the location, because it is super central, 10 minutes easy walk to La Rambla, right next to the Paral-lel metro station which also houses the funicular taking you up Montjuic. And you're right around the corner from Carrer de Blai, which is THE pincho (special tapas) spot in the city.
If you look on Google maps, you'll think you also have a nice lush green park right in front of the apartment, but in reality it's just a concrete plaza with no gras whatsoever (which is apparently a common thing for "parks" in Barcelona)
The drawback is that the location definitely isn't for faint-hearted romantics, it's very urban. To be more clear, there are always a bunch of homeless people lurking in front of the doors of like all nearby entrances and one morning we stepped out the front door to enjoy the sunlight and fresh air, only to be taken aback by the splashing sound of a homeless man relieving himself of the contents of his bladder right next to us.
Overall, I'll give this place a decent 3 star rating, because it was adequate for the time we spent there and fairly cheap compared to other rentals in comparable areas.