First and most importantly: skip this OVA and watch the movie instead, since this OVA is literally the same as the movie (this OVA starts at the scene roughly 15 minutes in). Not much to review about this. It's basically a gory, insect-rape doujinshi and it's pretty bad, even by that standard. With too much focus on awful cliched characters and a story that isn't even explained, unless you read the manga prior to watching, you're in for dull character interactions and several shots of insect rape/gore. It's harder to care about the story or characters when you get dropped in mid-story and it ends witha cliffhanger to the next scene.

In short, if you're interested in watching this for a story, skip the OVA and just watch the movie. If insect rape is your thing, there are a few short scenes for your viewing pleasure. Either way, I don't think it's worth it.