I watched Diabolik Lovers really on a whim after seeing a funny spoofing of the series on Youtube. What I found was exactly what I expected: pure romantic(?) kink wish-fulfillment. Because let's face it, this anime was made to appeal to an audience of young girls with a vampire fetish. It's purely a guilty pleasure sort of anime that I'm sure many of the show's fans can clearly see the flaws in, but enjoy it nonetheless due to a certain sense of misplaced romanticism and a healthy dose of hormones. All it's flaws are pretty easy to point out and I don't think I need to mention very many of them.
1. The characters: the boys are very similar to their female counterparts in other harem anime, each one being isolated to a different character archetype with little to no other personality. Their backstories may be their only saving grace. Yui is a generic main character if there ever was one with little personality at all. Which makes sense considering this came from a light novel.
Most light novel adaptations have very similar problems in the character department, especially in regards to their main characters.
2. The relationships. I don't even think I need to elaborate on how unhealthy Yui's relationships with all the boys are.
3. The ending. Mediocre at best, and a total cop out really. Not original in the least and exactly what everyone expected to happen considering it's a vampire romance(?) anime.
The only points I'll give in its favor really is the plot point about Cordelia's heart. Now, if the anime had focused more on that and less on Yui getting pseudo-raped at least twice an episode, then it may have been a relatively decent vampire anime. Instead, it dragged on way too long simply establishing each of the boys by having them bite and harass Yui a couple episodes before mysteriously losing interest to let another boy step up to the plate for the next few episodes.
I will be honest here and just say this is not a good anime and could have done better without the sadistic biting and the dragging story line.
Watch at your own discretion, you may be tempted to pull out all your hair.