This drama will remain one of my favorites. If you're unsure whether to start watching it or not, just give the first episode a try. It has tension building from the beginning, and from then on each episode is amazingly creepy and suspenseful.

What truly amazed me was the camera work. I don't who takes credit for that, but it was brilliantly done. I don't think I've ever seen a drama with such elaborate shots and transitions. It really added to the story and it was done in a really unique way. It was just so good.
Aside from that, it has to be the first time where I don't have any flaw to point out. There are some things that do need clarification from the writer/director, but even these felt intentional. There's a phone charger coming into play for a bit of the story, and this scene is open to interpretation, but I don't think they would have integrated it to the story if it didn't have purpose. Whether it be to add to the atmosphere, or simply to show a character forcing an encounter, I'm dying to know what the meaning behind this scene is.
The ending was also incredibly chilling. One of the major advantages of the show was the completely unpredictable future of any given character. That to me was amazing, and the ending really fit the story.

The Actors :
- This was my first time encountering Im Shi Wan, who plays the lead character. He was really impressive in his role, it really made me feel like the plot wasn't just a story. Like we were witnessing someone else's daily life. It really was amazing to watch him as the main character. You could sense the character's unease turning to pure fear, and the 9th episode will forever remain one of the most intense episode I've seen in my life.
I can't elaborate without giving away some spoilers, so I'll leave it at that, but he really made quite an impression in this drama.
- Lee Dong Wook was made for this role. I had seen him in Blade Man before this, and consciously avoided anything he was in after that. But to me this role attested his talent as an actor. It says a lot, because this has to be a really hard character to play. The chilling sensation you have when he appears on screen, this was a complete surprise to me. I feel like the drama wouldn't be the same without him playing Moon Jo.
Every actor aside from them was remarkable, and I had the pleasant surprise to see the girl from DAY6's music videos cast in this drama. It was nice to see more of her.

The story also had perfect OST to complement it, the songs really fit the drama, and were also amazing on their own. (Yes I am biased, The Rose are fantastic what can I say)

That said, if you have trouble watching bloody / gory scenes, this drama probably isn't the best thing for you. I advise you to skip a few scenes if you're determined to watch it.