This is tagged as " Comedy, Law, Romance, Crime, Drama" and it was, initially. There was a rather uneasy blend of crime and comedy, but the humor was quirky and the soundtrack provided a lot of good laughs. I initially hadn't planned to watch it because I hate most Mafia dramas (I went to school with children of real Mafia bosses, so it's not a world I find amusing) and because I'm not very impressed with Song Joong Ki's acting (bring on the haters!). Song was better than I expected but still not great - he simply doesn't have the gravitas I expect from a consiglieri. And I never felt any romantic vibes between him and Jeon Yeo Bin; they were pretty cold fish until the hastily tacked on end scenes.
But he real issue is that it went from a nice hidden treasure drama to a vicious revenge drama with over-graphic hate and sadism. There was nothing likeable about Vincenzo, who showed himself to be a purely evil as the psychopath he was trying to bring down. And what happened to all the gold? Was anything worthwhile done with it, or were the tenants just sucked into Vincenzo's world of revenge and greed? In the end, he corrupted them all.
The star performance was, of course, was Ok Taec Yeon, who went from a bumbling office assistant to an evil psychopath seamlessly and believably. It was a great performance. In the end, I felt sorry for him, a clearly troubled defective human being, while I disliked Vincenzo immensely. If you enjoy cruelty and revenge, you might like this. But don't be deceived by the lighthearted moments in the beginning - this drama is nasty.