I binge-watched it almost.
Ha Ji Won - 9.0 for acting. At moments I would expect more drama but still impressive acting.
Now Ji Chang Wook - I hated his character for poor acting, driven from lack of personality development which is writer's fault. I feel the drama would've benefited more from a more composed, matured emperor towards the end. There's very little manliness added and Ji Chang Wook's acting did not emphasize that.
Generally we are looking at a strong, smart, courageous empress and her even more admirable first love (King of Koryo) caught in a love triangle with an even quitter, coward emperor.
Towards the end, the empresses heart will lean towards her emperor but since there was no character development for the later, it feel illogical.
The plot is captivating, the cliff-hangers will keep you watching episode after episode. Around ep. 35-45 it will get more boring as the Main original conflict is over and the writers build up the story for a second conflict but it never reaches the impact of the first part. Everything is then wrapped-up in 2-3 last episodes leaving a feeling of " what was all the work through-out the last 12 episodes for?"
The ending is sad so it may leave you frustrated as such sad events towards the end were an opportunity in my opinion for actors to show the climax of their acting and characters, but instead it feels like a tentative to end the story abruptly and get over with it.

So: Reasons to watch:
- Ha Ji Won,
- Joo Jin Mo - Greatly acted, the role fit him like a glove, and OMG he so Over-Played Ji Chang Wook [but again the character and writers helped here... In my opinion]
- Story - line.
- Beautiful scenes
- Amazingly good portraying of dramatic emotions of two lovers that are destined to go separate ways.
- If you're into a sad romance - this is one of the best examples available.