Note: I've only seen this dubbed, so this may not be a 100% fair review.
Believe it or not, this isn't an average hentai. Instead, this entiire half-hour OVA feels more like a comedy with hentai scenes thrown in. From the quirky situations our leads get themselves into to the eye-grabbing messages thrown in throughout the video to seperate segments, everything about Kouin Tenshi feels humorous.
If you read summaries for Kouin Tenshi, you can easily tell that the plot doesn't sound like a hardcore hentai. The entire thing is just a project three schoolgirls just decide to do, that's all. And although the OVA feels unresolved at the end, maybe that contributes to the realism of the girls' little project.
Now, one thing I have to comment on is the animation. This hentai was clearly done on a shoestring budget, because the art is atrocious. When the girls aren't taping, the art focuses on close-ups of the eyes of each character or still shots with the occasional mouth flap. Oh, and one thing must be pointed out before you embark to find this OVA. The only male in the entire show has, let's just say, a member that looks a little out of the ordinary.
Honestly, I believe it would be hard for anyone to get off on this anime. However, if you're just in the mood for some raunchy adult humor and nothing else, Kouin Tenshi works perfectly. Good luck finding it, though.