I watched this without expecting too much because of many disappointed people who watched it before me and just like them I still watched it because of Ji Chang Wook and the synopsis didn't sound bad.
Soon I was proven right to not expect anything as the beginning was dragging already. We get a lot of insight into the life of the main leads which is not that interesting.
Then when the experiment goes all wrong and they wake up 20 years later you might think now it's picking up pace but instead we see overacted (fake) melodrama and comedy. The side characters are annoying, especially Ma Dong Chan's family and the female lead's ex yet they get a lot of screen time which is a waste. The funniest parts were those with Ma Seo Yoon (Ma Dong Chan's niece) or the female lead's brother which became my favorite characters beside the professor's aide. The main couple lacks chemistry in my opinion which was the least thing I could care about.
The worst was the draggy story with lots of unnecessary side and cliché plots. I was able to predict the end in detail because it was also cliché. They tried making the end exciting yet it only left a lame, bitter taste.
I feel sorry for the cast because I think they can do so much better and I don't blame them, but the script. Have nothing to say about the music as I can't even remember it so it surely wasn't outstanding. For me there's zero rewatch value, as it has more negative aspects than good ones.
So to make it short, I would recommend not to bother and go watch something else.