NO SPOILERS - This OVA surprised me. The plot was worked out quickly, but nicely. If you haven't seen "91 days" and you're not sure if you want to get into this anime, try looking at this OVA first. Yes, unfortunately it spoil you some important stuff, but above all, the most important moments from the second half of the anime don't show up.
I think this deserves a lot more attention. Especially if you love anime "91 days".