This OVA covers one of the missing arcs skipped in S1, and that is the Showdown in Barato. This is an essential arc in the manga for what comes after, despite the actual battle being a fight over tattooed skin. It can be watched before or after episode 12. For anime-only viewers, it might make more sense since it is self-contained.
Up until the credits cover the majority of chapter 55 in the latter part and the rest of volume 8, which is chapters 56-59. Overall, the quality is the same as most of S1. Animation is fine enough, though art direction is where it struggles. Though if you have been loving S1, you will really enjoy this too. I was hoping, since it was an OVA, that the whole episode would focus on it, which is not the case.
After the post credits, it adapts a missing chapter, which is chapter 32. While it is nice to have it adapted, this chapter is so rushed that it defeats the reason to adapt it. It feels like a throwaway in the anime.
I put this as recommended because most likely you watched S1 if you are looking for whether to watch it, so I would say definitely watch it for the “Showdown in Barato”. Though as a manga reader, keep the same expectations that you had when watching S1. Voice acting and sound direction are still the main highlights.