Disclaimer: This is not a site review. If you're reading one of these, it's unlikely I'll ever write a site review for this anime. Probably because I don't want to pick it apart. ^_^
I'll be honest. I saw Mnemosyne, because VivisQueen felt obligated to mention the large amounts of sex in her review. That said, I enjoyed the whole experience heartily. After the lackluster first episode, the OVA gains momentum until the final installment ends in a comparatively unimpressive manner. Maybe if there had been seven entries instead of six, the conclusion wouldn't have felt as rushed and confusing.
Story aside, I liked the side characters in the story a great deal. While Rin's studied cool and Mimi's willful childishness certainly formed the centerpiece of the series, I enjoyed watching the Maeno family grow more than the machinations of the principal actors. The immortal girls' lives touched many people and their growth really added meat to the narrative and fleshed out the world of the OVA. I think the focus on Mimi and Rin's relationships with others prevented this series from being a typically self-indulgent story of the supernatural.
Paired with decent visuals (points here mostly went to competent execution and excellent character designs) and an effective soundtrack (Rie Kugimiya, FTW!), Mnemosyne was definitely worth the watch. And the ending really was't as bad as I feared it would be.