I was surprised that there were so many bad reviews because I was really enjoying this drama. Literally couldn't stop crying during the latter episodes.
*****Spoiler****** this is a tragedy, it's not going to go the way you want it to, its frustrating and heartbreaking, but that doesn't mean it's a bad drama. I think the reviews that are critical of the leads' performance make sense but the ones that just rate this drama low because the ending was a tragedy I don't think I can agree with. There are plenty of happy ending shows out there these people can watch instead. And there's a difference between a bad ending that actually ruins everything and a tragic ending.
Overall, I enjoyed the plot and the characters. They all had good depth and really got you invested in them. That being said, I do wish the main leads were more capable/involved but that goes back to it being a tragedy. It's a futile fight against a very strong evil. So I can't really blame them.