The acting was rly spectacular. I personally think that Shin Hye Sun and Na In Woo nailed their characters.
Shin Hye Sun was amazingly versatile in showing both the differences and similarities (in the spinoff) between BH & SY, and her portrayal of Sobong's mannerisms and thinking expressions were really on point. Na In Woo did so well in playing the 'villain' that wasn't really a villain at the same time, who was so heartbreakingly selfless in his love towards SY.

For the story, I didn't love the ending, but it's to be expected for time-travelling historical dramas. I think that a lot of explanation is required for the ending to be satisfying and even then, it depends on an individual to decide whether they found the ending to be satisfactory. For me, I like to think of it like this (kinda to comfort myself):
-BH and SY shared the same body, which is why BH's body was in a vegetative state, and there was no swapping, nor was SY's soul in the lake.
-SY was there and felt his presence, and was completely aware of what was going on (when BH left, she said that he was gone, and didn't seem shocked about where she was or what was happening).
-I think it's just that BH was the dominant one the entire time, though as time went on, SY became more dominant (in later eps he could feel her hormones and her romantic feelings, and told her to back off, and it wasn't just him being dramatic haha).
-SY was hiding(as she said in ep20), and BH helped her become more confident (eg so she could swear in ep 20), while she helped BH change (when he went back his past had changed for the better). SY and BH have similar personalities(as shown in the spinoff), though BH is crazier.
-I'd also like to think that CJ fell for SY before BH was there (because of a scene in the spinoff), but he suppressed his feelings after finding out that SY was from the Andong Kim clan.
-Also, Kim Jung Hyun (actor for CJ) replied to a question in a live and said that it didn't matter whether CJ knew that BH was gone cuz it didn't matter whose soul was in the body, what mattered was how he chose to see SY from his perspective.

Anyways, I loved the comedy aspect of the drama. Sobong, her interactions the other characters, and crazy mannerisms, were the reason I started the drama in the first place.
Also, I love how Byeong-In was written and how they made me feel so sad for him. He loved SY selflessly from the start and was always there for her (eg in flashbacks, and before CJ cared for her in the early eps, though sometimes he was a bit overkill haha), plus how he sacrificed himself for SY and was killed by a hitman sent by his father. Additionally, I love that he was the only one to realise that SY had changed (tho BH told him he wasn't SY... but hey he realised that SY was still there, and BH didn't tell him that :"D)

Anyways, I think the ending is one of the happier options we could have gotten, considering the face that it's a time-travelling historical drama, though maybe they could have made us understand SY better with more flashbacks. Although the ending may not be 100% satisfying, I find the entire story and the episodes leading up to the ending so funny and enjoyable that I still think this drama is worth a watch.