We stayed at the apartment for a week, and it was okay. I think for a shorter stay, we would have rated it higher, but certain things were inconvenient enough that after a week we were so ready to get out of there.
Location: Super close to Mercat Central and the Historical City Centre. In that sense it's in a good location. However, note that there is a ton of construction in the area, so it's dusty and noisy in the neighborhood. The construction I'm sure makes it less charming than normal, but I think currently there are more pleasant neighborhoods to stay in Valencia, and you can pop into the historical center. Outside of the neighborhood the city is vibrant is pleasant, but in the immediate vicinity it's currently sleepy, dusty, and obstructed.
The apartment:
Pros: good a/c. Good blackout blinds. Clean.
Cons: bad wifi, bad key situation, very basic, ants.
The pillows aren't very comfortable, there is only a flat sheet on the bed (no duvet or quilt), and it's missing a few things that would be useful. There was no garbage bin in the bathroom, and when we asked for one we were told no. (Again, we were there for a week so this got annoying). Also, there was no wine opener. It's Spain! The wifi didn't work for 2.5 of the days we were there. Restaurants and cafes in the area don't offer wifi regularly, so it was a pain. The wifi isn't reliable enough at all for anyone who has to work from home. The router is in a different unit and it's only an extension that's in the flat.
The biggest issue we had with the place was that we only got a second set of keys a couple of days before we left. In order to leave the apartment, you had to unlock the door with keys. So either one of us was locked in the apartment, or we would have to coordinate around having one set of keys. This was terrible. It was very hot, and there were a few times that because of a lack of internet or whatever, one of us would be either locked out or in. I had to ask many, many times before they finally agreed to give us another set.
The last thing is that there's an ant problem. It was very hot a couple of the days we were there and those days, ants came in. We were able to deal with it ourselves, but just be aware it could happen.
Check-in: We received an airbnb message with checkin instructions and they were a bit confusing. The address was wrong (use the one in the app).
To sum up, we'd go back to Valencia but we'd choose a different place.