The apartment has many pros and a couple of drawbacks. Ideally, I would give the apartment a 3.5. I would probably stay there again, but there were definitely some issues.

Positives: First, the apartment can easily fit at least 6 people and was suitable for our needs. For our group of 5 it was more than enough room especially given the two full bathrooms. The apartment is also located in an incredibly convenient location and it is a minutes walk from La Sagrada Familia and you're a 10 minute taxi from basically any major attraction. There is also a subway station right in front of the apartment and hailing a cab was very easy because it is on a busy street. The apartment also made a good use of the space and the common space was large enough for all of us to be in the room. Also, the host was super helpful and available 24/7. In fact the office allowed us to print off our boarding passes at 10pm on a Saturday night!
Donwsides: the apartment is a bit old and the walls show it. There is just a lot of dirt on the walls throughout the apartment which definitely brought down the aesthetic. Also, the rooms are tiny, but functional. However, one of the rooms did not have any storage space for our clothes. Also, the walls in the apartment building are incredibly thin. Upon entering the apartment, there are numerous signs that require guests to be quiet while in the halls because the walls are so thin. Also, there is a no-noise policy at 10pm every night because the walls are so thin. It was a bit irritating to wake-up to the sound of your neighbors conversations. Moreover, the water pressure in the bathrooms was way too high and the floors would become soaked after a quick shower. Also, the water pressure in the sinks was too high and it would spray everywhere. Lastly, the mobile key entry was rather temperamental and unreliable. We often spent a couple of minutes trying to get the door to unlock because the keypad would not cooperate with the app.