Old but golden, a true classic.
It's got all the features that make older sageuk/ancient dramas more enjoyable than newer ones (elaborately designed sets and costumes for example) but also quite a few faults, the main one being the characterisation of the titular character. Dong Yi is a Mary Sue, pure and simple. She always has ideas that no one else does, notices things that no one else sees, suceeds where everyone else fails and goes scot-free in cases where anyone else would be punished. Now I understand that the protagonist of a story usually has more grace than other characters but this show takes it too far. The worst part is that Dong Yi herself isn't a particularly intelligent person —she often makes straight up stupid decisions but suffers no consequences for them because of her main character halo— so oftentimes, other characters are dumbed down to allow her shine. The plot also drags quite a lot as is often the case with long dramas like this.
All in all, a 7/10 drama.