The story is involving and entertaining. It's got palace intrigue, great enduing passion, a love triangle and lots of tears and grand drama. The mysticism is interesting. There is a suitable musical score and the costumes are nice.
The first 5 episodes the child actors carry the show and they are quite good. When the grownups take over I didn't like the lead actress - she acts quite well but her face is too round. If someone more beautiful had been cast it would have been better. But the story is involving so it doesn't matter. Kim Soo Hyun became a star from this show. As the King he is watchable. Sometimes he slightly overacts but I guess it is the story that makes him shouts and cry a lot. Only thing I didn't like was some of the self sacrifice.
If you are reluctant to try costume Korean dramas this one is actually a good introduction. Between the light hearted Rookie Historian and this one I prefer this one. It was a huge hit and there is a reason why.