DISCLAIMER: Rental Girls is not for the faint of heart. If you do not enjoy or want to see lewd character behavior and heavy sexual content, such as Borderline H or Explicit Sex, then I suggest that you do not read this manga.
The straight forward answer simply put, sex. Rental Girls is essentially a bunch of sub-stories about the reader (or viewer) going into an adult video store and renting movies. The movies are what come to life on the pages of the manga. That about sums it up. To be fair though, the stories (or movies) themselves are each different and have more depth to them than any "real" pornographic or obscene video has to offer (I would image, 😉). Although a repetitive formula is clearly being followed, at least the story arcs are differente from each, albeit still all about sex.
Consistent quality in every chapter with very detailed characters. It's more like a 9.5 instead of just an even 9, but just short of a perfect 10. Had the range of color vary evermore slightly and had some of the written "sex" sounds been reduced, which nearly cover some pages in their entirety, then it would be a perfect 10! All those excessive "sounds" make it seem like hot sloppy sex on a Slip 'n Slide. Overall, they are minor complaints, but still worth mentioning.
Lewd but with some personality wrapped up in each individual story arc's characters. Most importantly, the female characters are detailed quite wonderfully. They are all smokin' hot right from the very beginning, at least as far as manga illustrations go. The "no-name-girl" in the first story arc (ch. 1-6) is a gem, especially her actions at the very end. Professor Park from the second arc (ch. 7-15) and the real estate agent in the apartment arc are both quite possibly the best looking females in the series. Plus, the Daughters Adult Video Rental Store owners are easy to gawk at too, especially Gukhee. For visual, sexual appeal, Rental Girls' characters offer plenty.
A unique point of view is presented in how they tell the story. The reader is made to feel like they are a direct part of the manga; as if they are the first person POV protagonist themself. That is actually really cool and unique, but can only work in certain storyline situations and types of manga. This being one of them.
ENTERTAINMENT SCORE (2X): 9/10