The apartment is nicely decorated, the bed is comfortable and it's equipped with all one can need. It felt clean and had AC even in the room, which is nice in the warm summer days. For us the location was good as we were going to a conference in the Alfandega congress center. It's also pretty close to the city center, with lots of restaurants nearby (it was difficult to find grocery stores though, it was surprising). They do gift you a bottle of red wine which is a nice touch, and it was actually an ok wine! (we were expecting it to be terrible/cheap wine, but we were surprised).
Cons: the aparment is uphill although there are escalators to get there. They're supposed to work 24/7 but in reality they turn them off at night (8pm or earlier) so be prepared to walk up a lot of stairs if you come home late.
We had a small issue with the apartment: on our third day there, the electricity went off. We called GuestReady several times (first time was at 16h-ish) and got the problem solved by 21h-ish. Basically we had to move apartments cause the issue was with the provider and they couldn't do anything. It was a bit annoying having to move out in the dark and having to carry the suitcases all the way down those stairs with no help. The other apartment was bigger but also older and didn't feel quite as nice, but I guess I'm here to judge the first apartment only...
On the plus side, the waiting times on the phone weren't too long, the calls were free and they did solve the issue in the end, so you can trust them. They gave us a choice of apartments, which was also appreciated. If it wouldn't have been for that issue, we would have been 100% happy with our stay. Unfortunately it did happen in the worst possible moment for me, as I had a presentation the morning after which I wanted to rehearse, and I also had time-sensitive work to finish (which I made them aware of). I would have probably expected them to offer some sort of compensation or discount on future stays for having had to find someplace else to work (it was a bit of a stressful situation for me), but they didn't offer anything other than the relocation, which was a bit of a shame. Overall though, I would book with them again.