If you want to watch the series then this will be a lot of "extra" content for fans. A lot of time is spent on flashbacks, yet those flashbacks are extended and actually explained in the series (as flashbacks) leaving the rest of this ultimately as an extended side-story.
If you're not getting hung up on the details of who's who and what happened back-then, this works as a standard OVA.
If you want to get into the series, feel free to skip this.
Explanation by cesu on Friday, 03.05.2013 02:06