I woudn't want to be one of those 'i only liked the first season' people. And I'm not. The second season was narratively excellent.
While I waited for s3 to have a satisfactory concusion, the writer went for cheap shock value. t didn't deliver on the characters. Not only it wasted time with secondary characters and plot points that did little to the story, like Beak Jun Ki, it failed to give much needed detail on the primaries' motivations, like Su Ryeon's and Yoon Hee's .
I am saying all that as the biggest fan of the series. I liked it until s3e12.
Not to mention the finale. In my opinion, it reversed the whole point it had established for three seasons and betrayed the characters and the audience alike.
So should you watch it? If you've watched until s2, it's impossible not to. But don't le the awful ending negae a year long lovely experience.