The central appeal of Rated R, for me, is Lilly the co-protagonist. Lilly is a seventeen year old girl who works at the last remaining video store in California (which is actually a front for the mob) and is pretty much the distillation of my own teenage self's fantasy girlfriend. She's a goth, horror fan, troubled but cute, and perfectly willing to date nerds as long as they do her Calculus homework. She was originally the cover character on the book but I suspect she might have been taken down due to her age.
Much of the book takes place from Lilly's perspective and these are the best parts of the book. Lilly has an entertaining personality that completely lacks respect and is a bit Buffy the Vampire Slayer only with more swearing. While the book is a loving homage to 80s action and horror, it has the kind of self-awareness which came from Joss Whedon and Wes Craven's "Scream" movies.
Co-starring in the book and infinitely less interesting is the character of Sid who serves as a nice deconstruction of "monster boyfriends." He's a demon-possessed teenager (or something supernatural--it's never entirely clear) that was raised in a cult compound to be a slayer of monsters as well as people. Sid has never experienced normal life and is making a very poor attempt to fit in by getting a job at Lilly's video store.
The joke being Sid is poor, maladjusted, and only good at killing versus the typical idealized vampire or other hero. Albeit, his super strength does lead to a hilarious scene where Lilly does think he's a vampire and begs to be turned in an overwrought drama that just leaves Sid confused. It's also one of the funniest scenes in the book.
A warning that Rated R doesn't pull any punches and could easily offend those of delicate sensibilities. The villains are monstrous psycho killers, rapists, and torturers. Their actions are done off screen but this is not a book which shies away from the horror elements of the setting. Lilly is, shock of shocks, a sexually active seventeen year old as well. Which, wow, who could have imagined such a thing existing.
If these things don't offend you then you'll be in for an excellent ride from start to finish as the book is highly entertaining fiction written for adults. It's a gory fun celebration of the kind of movies which fourteen year old boys in the late 80s and early 90s rented from the video store because the clerks didn't care.