Hands down, drama of the year (perhaps tied with Crash Landing On You though XD
I had high expectations because this is Kim Soo-hyun's long-awaited comeback after his military service, and let me just say the cast, the story, the production have all exceeded my initial expectations by many folds.

Story: It is marketed as a dark, healing drama from the start, but while the characters have a dark history and are working towards overcoming their trauma, the plot is full of lighthearted comedy both in the characters as well as the cinematography. Because the main character is a children's book writer, each episode is also centered around a fairy tale theme (and if none exists in the real world, the drama created a few very interesting fairy tales to fit the storyline too). Thus, the story is filled with references that will make you rethink what fairy tales are and what message they mean to convey. In particular, the fairy tale of the first episode really highlights what this drama is about -- learning to live with your most traumatic memories as a means of growing up. As a story about growth and healing, the character development is phenomenal... the script touches upon multiple concepts of self-identity and values, and the use of them as motifs driving the story progression was A+. While the villain is truly scary and bad and I usually hate villains, I just couldn't stop admiring how much the characters added to the story-telling. Everyone had a purpose -- no matter it was to drive the plot or to make us laugh -- and boy did we laugh, HARD.
Also, this is a masterpiece from the literary point of view -- as I said, the self-identity/values/worldview motifs that keep coming back really drive home what the scriptwriter wants to convey through this drama. Many funny lines / scenes in early episodes also keep on coming back in different shapes or forms, and the way the scriptwriter used them as comedic relief has me down on my knees.

Cast and cinematography: The lead characters AND the supporting characters all added flavor to the story, and everyone suited their role so well. I'm particularly a fan of the publisher duo -- they were so funny! (If you see Park Jin-joo in a drama, you know it's going to be hilarious). It was my first time watching Oh Jung-se (Moon Sang-tae) in a lead role, and his (as well as the child actor's) depiction of someone with ASD is really touching.
As the story is about fairy tales, the production really took a liberal approach in cute animations, outrageous special effects, as well as lens filters to create a "story-within-a-story" feel to the drama. The animated leads are so cute! Honestly, even if you aren't sold by the storyline or comedy (which I think you should XD), I would highly recommend this drama just for the acting and cinematography as well. The artistic value of this is BEST.

Music: Right music at the right spots; really adds to the drama. I would put the OST itself on replay. Enough said.
Rewatch value: Sooo many scenes that could become instant classics to be watched again and again. I look forward to when other kdramas start referencing It's Okay to Not Be Okay. It's going to be as meme-y as Secret Garden was 10 years ago I'M EXCITED.

TL;DR: It's a drama I would recommend you watch by yourself, with your fellow fangirl friends, with your significant other, with your family, with people at work... with everyone. Definitely something you could talk and rave about for the next few months (I know I will)