Not only is the plot formulaic, but it's also very predictable. So you have a bunch of people in a lockdown building and they have to survive for 7 days. Basically the plot of the ring and saw blended together. Then we're introduced to a character, we get a flashback, something happens to said character and repeat the whole process.
I will admit though, I couldn't stop reading until the end. Despite its repetitive structure, it does know how to keep the readers hooked. However, I don't think this is the kind of manga you'd want to re-read after finishing it. Once the manga ends, it just... ends.there was no catharsis by the end of it. But for the sole reason of keeping me hooked 'till the end, I still gave it a decent score. I'd score it 5.5, but MAL would just round it off to 6, so there you go.
