Yes, the story is good. Yes, the smut is good. If you had to choose between the R15 or the R19 version, the answer is to just start reading. I started with the R15 (manta version) and I just love the intense pure love that our two main characters have for each other. It is PURE! (And yes, they do have a lot of sex. I did not realize how much and how smutty it was until I started reading the mature version... Either version is great to pick up because the all-ages version is chapters/a season ahead of the mature version, versus really great smex scenes.)
The characters are so lovable. Part of the appeal is that the two main characters wholeheartedly love each other. There is no doubt in any part of the story that Jaehee loves Dojun. And early on, it's clear that despite Dojun avoiding Jaehee, he too really likes Jaehee. They love each other- a lot and it feels nice to read without any love triangle or misunderstandings that other romance stories typically have. I'm also fond of how incredibly book-smart, yet naive Dojun is, and how rich and single-minded Jaehee is. While Jaehee is obsessive, at least he's not two-faced or socially manipulative like some other romance characters.
Unfortunately, flashbacks in stories are not always the easiest to implement. Season one of 4 Week Lovers goes between the present and past more than once and unfortunately, it gets -0.5 in its rating from there. We get a lot of information about the characters, especially Dojun, through the flashbacks, but we're initially introduced and invested to the story in present times. Flashbacks, especially multiple flashbacks, can make it harder to enjoy the story.
The story and the characters are still really, REALLY GREAT. I HIGHLY RECOMMEND READING THIS. The switching between present and past times only really happen in season one. Keep reading and you'll definitely enjoy reading this comic. Wholesome love like nowhere else. And the panels in smex can get interesting, as well as the smex itself.