PS: I watched this long ago but I'm only reviewing now
This might not be for everyone due to it's horror sageuk genre, but for me, this drama is really good. Except for a few scenes, I don't find this drama scary in a general sense. The effects used may have aged a bit, but I didn't really mind.
I really love how this drama used Korean folklore to create this original story. I also love how the fantastical elements in this story are used to show the themes of this show: mainly about parents' love for their children and the lengths they'll go for their children. This along with Arang and the Magistrate are both good kdramas that used the Korean folklore very well to tell their story. This drama about family. It's also a drama about children and their adventures, rivalries, and early experiences in romantic crushes/love.
The actors are really great. Han Da Gam (known as Han Eun Jung when she made this drama) was amazing as the Gumiho mother. She also has great chemistry with Jang Hyun Sung (actor of her child's rival's father who fell in love with her character) and Seo Jun Young (actor of the young male servant who also fell in love with her character). Kim Yoo Jung and Seo Shin Ae are both amazing in here as the two main children. Kim Yoo Jung's acting as both the sweet human child of her gumiho mother and her scary gumiho version is really great. No wonder she is one of the most well known actresses in Korea today. I still think this is her best acting in all her acting projects that I've watched. Seo Shin Ae is terrific as the wicked and spoiled child of rich parents, you'll really loath her character. Sad she's so underrated, I really believe she deserves more great acting projects. Lee Tae Ri is only in supporting role, but he's great as the teenage Jung Kyu, the young boy in the affections of both Kim Yoo Jung and Seo Shin Ae's characters.