I have had this title on my watchlist for a long time now and was excited to one day give it a try after seeing all the high ratings and good reviews, all which seemed to be solidly justified by the interesting premise of the show.
The first episode was interesting and got me hooked. Loved the main OST title ("Hallucination"), although after being overplayed at every single "dramatic" scene with flashbacks from the past (which was like 20 times per episode) it got rather lackluster. Loved the switch between personalities and was impressed with Ji Sung's acting, as this was also my first time watching one of his shows. Despite not initially liking the FL (and I don't understand exactly what it was, maybe the hairstyle, but it looked so weird to me when she was fighting with her brother and him mentioning he's older than her so she should call him oppa because to me she seemed way older than him like they were noona and dongseng), I was hoping it would get better. Oh boy, was I wrong.
The writing for this kdrama was so generic and bland, it seems the only times the screenwriters took any steps away from the traditional kdrama recipe was when the other personalities of the ML were involved, but ultimately only Shin Se Gi's actions actually had any sort of impact on the plot. The original ML personality was pretty boring and felt like he had little to no character development, along with the FL. The love story was also bland, full of tropey scenes that instead of bringing comedic relief, only managed to bring the quality of the drama even lower, once again relying on the "childhood connection trope" which is obviously enough for 2 characters with very few meaningful interactions to become soulmates (/s).
There were no real resolutions regarding the 'villains' of the drama and to be honest they were so unmemorable I already forgot their names.
Her brother was such an unecessary character I don't think he was in any meaningful scenes in the last few eps. And his storyline with being a secret writer didn't add anything to the main plot. Not even mentioning that at some point it's revealed that he's actually in love with the FL.. wtf was that? I know they're not blood related, but still, they grew up as siblings.

Overall, if I were to make a pro-cons list, it would look like this:
pros:
+ very entertaining acting from Ji Sung taking on 7 different personalities (more specifically impressed with Shin Se Gi, Perry Park and Ahn Yo Na).
+ Shin Se Gi everything
+ the main OST.
cons:
- sort of everything else.
Overall I wouldn't recommend this drama unless you're a big fan of Ji Sung or you're not necessarily interested in a good watch, just something to have on the screen when eating.