Overall it's a nice place. Great location, within walking distance to the most recognizable parts of the old town, the Turia parc and lots of great eating places. A bus stop which will take you the the City of Arts and Sciences and the seaside is 2 minutes away. The apartment is clean, and while we were there the street was quiet apart from occasional cars passing, and the neighbors were reasonably quiet too.
Having said that, there are things that could be easily fixed but were frustrating in the long run.
We found the double bed quite uncomfortable, with a mattress that was uncommonly hard. The worst part though was that the bed also sqeaks terribly with every toss and turn, so if you or the other person move in your sleep, you better be heavy sleepers, or you're going to be waking up repeatedly during the night like we did. Additional, softer pillows would have been useful, too.
Also the shower seemed to be clogged, and the drain quickly started to run very slow. It got frustrating enough that we tried to unclog it ourselves - however it proved impossible without unscrewing the drain, which we didn't have the tools for. Also, the gasket on the door was already broken when we arrived in the apartment for the first time, and half of it slid out from the door into the shower tray, and would slide out when we tried to put it in place. This of course made the door leak water. And one meter of gasket lying under your feet when showering can be annoying, too, especially with a clogged drain. It would be impossible to miss if anyone took a moment to inspect the apartment, and would be easy to fix - preventing guests from getting a bad impression.
Last thing is that there is no place to leave your luggage near the apartment after you check out - if you need one, you'll need to find a paid service on your own. Our hosts pointed us to a place nearby that was supposed to run a luggage room for free, but the place appeared to be out of business.
So here it is. Overall we had an enjoyable stay, if with a few annoyances that could be easily fixed. But we had a great time in València nevertheless.