I think this is just the typical case of an unnecessary special/OVA. The episode focuses on Sato's past, and there is nothing that you either already don't know or you can't already imagine from the main show. And Sato already has a problem of being a one note character, which this OVA furthers even more. It just feels reductive and repetitive. Overall the technical quality is the same as the main series, and if you super love Ajin you can give it a shot. It's only 1 normal episode lenght, so it won't take too much of your time. But overall I think it wasvery boring and skippable.