This show grated on me from the very first few minutes, but I was determined to persevere because of its fame, (and because it was recommended to me after watching the absolutely amazing It's Okay That's Love) but I have given up. Here are a few reasons why:
1. Mental health:
This is not a show about actual mental health issues. There is an evident lack of sensitivity (and sense), and the Multiple personality disorder is treated as an entertaining plot point. Mental Health illnesses are not comedic exaggerations of personality quirks. If you want to watch a tactful show about mental health, watch It's Okay That's Love, and don't touch this show with a ten foot pole.

2. Characters:
It does not matter how well Ji Sung could switch roles. In the end, it was a badly written character and no amount of acting can do justice to it.
Hwang Jung Eum: This lady is probably the one with DID, with how different her acting was in Secret, and in this one. Her character was like nails on a chalkboard from the moment she uttered a word in this show. Good lord, what adult woman, and a psychiatrist at that, behaves in such a teenage juvenile fashion, screaming at a high pitch all the time? Maybe it is a fault of the writing and not the actress, I'll give her that. In any case, that is not the demeanour of a psychiatrist and it instantly felt like cheap entertainment to me instead of a show aiming to bring awareness on mental health.
I have nothing much to say about any other characters.

3. The Chaebol grandma storyline: So cliche, when will K Dramas stop doing this.

I could go on and on, but even if my first reason gives you pause, then it's best to not watch this show.