The concept of lovers being connected by some tragic event before they started dating, is at not unique, but it is for sure satisfying when done right.
1-First I have to say that I much prefer the past connection of the couple to be something that happened during their childhood and not something from 500 years ago. This show proves me correct, you do not see the character being affected by the flashbacks from past lives, but they actually live with the traumas, emotions that when they were together in the past, just make more sense.
2-Another thing plot wise they did very good, is having the love triangle(something that I always feel like most kdramas fail) is having the other love interest not be a serious love interest in the first place. It just makes the main couple that much warm and rich.
3-They acting is great! I already loved the main actress from other dramas and the main actor was very good too. Even secondary characters, like the maid was very entertaining and left a strong impression on me.

Overall, I think this show presents a story that is "surprising" but it amazingly crafted, making it a great watch that I don't think anyone would be disappointed.