This Fafner prequel takes place a few months (?) before the events of the TV series and revolves around a last-ditch effort to keep the Festum from discovering the location of Alvis. While watching the show, at first I thought this was kind of a stupid story idea since -- if you've watched the TV series -- you know that the Festum are going to find the island eventually anyway. But as it turns out, the inevitability of failure makes for some tragic drama, and in the end I thought they made a good explanation for why achieving peace -- however temporary -- is so important. So all-in-all, the story is pretty good -- not great but worth seeing if you're a fan of the show.
One thing I was really disappointed by was the fact that it doesn't do much in the way of providing insite into the Festum or Fafners and why they do what they do, especially since there is a lot of stuff in the series that I still don't understand. But... Eh..
The visuals are a definite step up from the TV show. The Festum themselves are rendered in greater detail and the battle scenes are a lot more fluid. The music in the OVA is good too, but I don't think it's quite as good as in the show. Angela has a new song in the soundtrack, so that in itself is something worth listening for.
Overall if you are a fan of the original series I'm sure you're going to want to give this extra little Fafner episode a try. Though if you are not a fan, I don't think this will change your opinion one way or the other.