Overall, I would definitely stay at Quartprimera apartments again!
There are a lot of pros to our stay here. The first thing I look for in a hotel/apt is the location. Quartprimera is located on the same square block as Casa Mila, right off the street of Passeig de Gracia. Thus, it was convenient to walk to the Old Town (around 20 minutes), take the metro (the station is literally less than a block away!), and walk around Passeig de Gracia/Diagonal where Gaudi's houses are. There are plenty of places to get food nearby: restaurants, supermarkets, and convenience stores. We went when the Catalan riots were happening and we still felt very safe while inside the building. Very quiet/peaceful.
The other things I look for are cleanliness, comfort, and spaciousness. The apartment was definitely clean (no bedbugs, rust spots, stains, dust, etc). We stayed in the penthouse apartment which comfortably fit three adults. It had two bedrooms (two beds total), one bathroom, and a very spacious living area/kitchen, all stocked with amenities (towels, shampoo and body wash, tea bags, cooking equipment/supplies). The washing machine was located outside on the rooftop patio, where you can see Casa Mila across the block.
The only con is that mosquitoes would fly in through the windows when we wanted fresh air. I realize that window screens aren't really a thing in Europe and the air conditioning worked fine, but perhaps they can install them so that the air circulation could be better.