Prior to this, I had the misfortune of watching a couple of average dramas which were disappointing overall. So this one was a breath of fresh air and very welcome.
THINGS I LOVED:
>Story: The idea of a girl and her family getting trapped in a creepy cult is not something often seen in kdrama land. It was very interesting. The pacing was good and the cliffhangers were well-placed. I also loved that there was a strong female lead. I'm also glad that there was little to no romance since that would've been completely out of place. I also like how the drama shows the gritty gruesome reality of bullying among males in particular and what it leads to. The story is also very realistic in the sense that the cult followers don't just snap out of a spell when their cult is exposed, but they hold on to their beliefs much more strongly when confronted with the reality.

>Acting: Seo Ye Ji's acting is amazing. Her fear, her outrage, her doubts, her mental breakdowns were all too real and very believable. She made the character come alive and turned into Sang Mi. Another actor who wowed me was Woo Do Hwan, who completely fits the bad boy-but-soft-inside vibe with his looks as well as his expressions. And his expressions were so on point! When he cried, he cried with his face and his whole body. He completely stole every scene he was in. I also loved the supporting characters, and the main villain was very good at making me hate him.

>Characters: I loved the strength of the main female lead. She fights for her brother, and then fights to keep her family hold on after his death, and then she fights against the cult till the very end. She's sharp-eyed and quick in reacting. She can keep a poker face and her wits when faced with life-threatening moments. She has her tender moments when she cares for Jeong Goo and mourns his death. Even when asking others to save her, she does more than half the saving by herself. The friendship of the male characters was nicely portrayed, and the few scenes containing humor were well-done though I'm glad there wasn't too much of that.

THINGS I DIDN'T LIKE:

>Loose/Incomplete Plot Elements and Details:
(1) I wish the drama had shown a little more backstory to the main villain, Baek Jung Ki, and his relationship with Jo Wan Tae. There should've been a few flashbacks to show that they'd done this cult thing in other towns as well, and how they'd wrapped up and gotten away before being convicted. Baek Jung Ki's mental state is not pursued fully, and there was potential in that. They could've shown him as a person suffering from DID or a plain psychopath.
(2) The mystery behind the holy water blessed by the cult leader wasn't taken to it's end. Was it drugged as well? Was it just a placebo? I mean it worked instantly on Sang Mi's father so I'm assuming at least some kind of hallucinogen was involved?
(3) The dead female reporter's body was never found, nor was Sang Mi very grieved to hear of her disappearance. I mean, she was a super strong ally, even though she was not very close to Sang Mi as a friend. Her boss's reaction to her death wasn't shown either. It made the whole reporter plotline seem very irrelevant. She didn't get any concrete evidence and Deong Cheol was way better at spying and ferreting out evidence than she was. She was careless enough to get caught as well. So I guess I didn't get the point of her being included.
(4) I didn't really get the point of the plotline with the ex-convicts/thugs and what their motives were. Also, the scenes involving one thug taking on dozens of armed men was just too cliche.
(5) The last episode was a bit of a let down. Sang Mi's mother meeting Sang Hwan's mother by chance when the latter was brought to Guseonwon, and the sudden immolation of Baek Jung Ki (although ironic and apt) was a bit hard to swallow.
(6) I'm surprised Sang Mi's dad didn't go along with Apostle Kang in her new cult. Rather than a loner, he seemed like an eager-to-serve slave who likes to remain in the shadows of the leader and become his arms. It would've made so much sense for him to latch onto Apostle Kang and do her dirty work in the new cult.
(7) Sang Hwan's mom regains control of her body in the nick of time. Now you're stretching my suspension of disbelief.
>Acting:
I don't know if I'm the only one, but Taecyeon's acting in this drama sucked. He had the exact same face when shocked, grieved, disappointed, thoughtful, heartbroken, or angry. It was the same face. It was disappointing.

>Character:
Again, Taecyeon's character, Sang Hwan was dull and naive, especially when compared to Sang Mi's. He was too trusting of his father, he kept making mistake after mistake, and he seemed to rely too much on his friends. If Sang Mi was sharp and quick-witted, Sang Hwan was the opposite. I guess you can say it was like a gender role reversal, with the male being naive and slow to act and the female being a bold character. But Sang Hwan often seemed a little childish.

So yeah, I enjoyed it but there was definitely room for improvement.