There has already been dozens of review written on this popular k-drama so I'm not going to go into great details about the bulk of the show other than to say that it was amazing, funny, sad, heartfelt, thought provoking, exciting, etc. Definitely worth the watch.
Superb acting by the FL and the ML was great as well. Oodles of OTP chemistry. Support cast was top notch. We'll just say no more about the palace intrigue plot, it was integral to the story but nothing new.
Now, some of the reviews focused on THE ending. I understood their sentiments but I don't agree with the more extreme views that the show was ruined by the ending. Yes, it was a bit of fancy foot work on the writer-nims' part in last 30 minutes but to do it otherwise would be to create and ignore longer term problems. You can't really have a 21st century person living the rest of their natural life in the Joseon era without turning history into swiss cheese. Even the few month he was there was already throwing up narrative issues that will make you cross eyed if you think about it. The problem is that most of the negative review all centred around the point that the 21st century person should be allowed to stay on and live happily ever after in the past because he earned it. Is it possible? Viable? Butterfly effect? What happens if he dies in the present?
The writer-nims did their best to push the two threads back together and it was done in a fairly light hearted and conclusive way. Let us not forget that the main relationship between the OTP was getting messy and confusing and it was sustained by the "true love conquers all" motif.
Other reviews also assumed the FL's soul was completely banished and it was obviously not true. By the half way mark, you can see glimpse of her old self popping to the fore. My reading of this is that the FL's soul was always there and can see, hear and feel everything but it was in a passive voice. So the original FL was a "silent" partner and fell in love with the ML along the way, possibly more so. She might even had taken the lead (subconsciously) at some key moments (you will know what I mean if you have watched the show). So it was not that much of a stretch for her to carry on once the 21C person departed. Her later comments and actions all pointed to her being aware of everything that had happened. Her eventual persona also changed a bit so it was a two way street. On the other hand, if they let the 21C person stay on then another segment of the audience will rail against that ending because of all the timey wimey and interpersonal issues.
It was brave of the writer-nims to go down the time travel/body swap theme and it was a deep rabbit hole they went down. At least with this ending, we can have the unicorns and a workable conclusion that I can live with and still smile at the happy thoughts.