no spoiler part:
It's really great drama, interesting, good twists and turns, high production value, good dramatical plot and acting, multilayered characters, good and interesting romance (plus almost every time romances/triangles are predictable, bot not here) and quite sharp emotions. though there are little plot holes/unrealistic details, but this is compare to overall drama level, otherwise 90% dramas have bigger plot holes/unrealistic moments.

another interesting thing about this drama - ML is typical anti hero SML, so if you love second male leads and/or anti heroes (like me, it's like real syndrome, almost everytime liking SML) this drama would be special for you. I loved that SML was good developed and had similar or more screen time, he is more ML type character, and also we could have damaged full of pain anti hero (very SML type) as main lead and because of that drama was more understanding to him, though he really has dark side in both lives, and this dark side is quite deep, not usual "kind of dark ML".
both male leads have their charisma and "SML charm" and because of that I wasn't sure who I rooted more. though drama manages most of things/plot developments good, I think at some point it was misleading and to say more about that I sould give some spoilers.

spoiler part:
in romance part there was something I didn't liked, as I said I liked that ML was SML like anti hero and it should help me to root him more, plus I kind of knew (think I read somewhere and the fact that he is ML) he would end up with FL, and of course in first life he so tragic viewer wants him to have something good, some happyness at list in second live.... but... at some point drama makes you root to SML and FL, they are like romeo and juliete, loved each other so much in first live and have so good prelude of romance in second life, though I didn't like this actor his character was so cool in second life, and FL falls for him and al this is so natural, good acted, they have chemistry, when she looks at ML as younger brother who needs support... Viewer feels pity to ML because his misfortunes repeats in second live, but as the drama was at list for 10 episodes, it was logical that FL loves SML and they are destined...
So I don't understand sudden change in romance part, why was she so understanding to ML, ok she is good person and sees he needs someone to support him, that he is damaged but not evil yet, but there was too much facts against ML and after all she believes him and supports anyway, it's not natural, when she didn't forgives SML (who she loved and said to him always be with him, never break his hart) because he shot ML...
understanding/forgiving everything to ML and almost nothing to SML is kind of old cliche and even this drama has it, but in last 4 episode suddenly she falls in love to ML? falls for guy who she hated in past live and looked as brother (plus she tried to stop him to protect SML) in this live, why? she was saying to him that her love to SML wouldn't changes, why they developed that strong chemistry/feels with emotional past life paralels if they were going to that ending?
I just couldn't understand this...