A lot of people seem to hate this drama, but I'm here to give a very neutral opinion of it. I neither LOVED it, nor HATED it.
I feel like this drama will really appeal to some, and others might not like it at all. It's basically all about your preferences.
What I liked about this drama? The characters from Korean folk tales. As a person interested and fascinated by the Korean culture and folklore, this was a treat for sure! Various mythical characters were lifted right off the folk tales and these stories were spun around to suit the story in a modern setting and incorporated beautifully in the plot. I also loved the cinematography, the food shots, the friendship between the different characters. Loved the relationship dynamics between the side characters, and the relationship shared by them with the main character. And Lee Rang, Sin Ju. That sweet baby. Aah, basically all the characters.
What I wasn't a fan of all that much? I personally felt like the main leads could have made better choices, that could have prevented a lot of trouble from happening, but hey. There will be no drama without that I guess. Also, certain sub-plots were under explained, like what happened to Ji Ah's parents in the end? The mystery of the island was not really fully, properly explained. How did the Imoogi get all those people to come there at the same time?
All in all, an enjoyable watch. I kinda felt like it was a bit draggy in the middle, but episode 15 and 16 were a total rollercoaster. They did end with a bang!
Would recommend watching this drama if you're interested in fantasy and folklore. If you're looking for too much logic and less sap, this is not for you.