I had to idea what to expect but was surprised with how good this was. The male lead delivered an outstanding performance -- in a couple of series I've seen him play characters with personality disorders or several characters at once, and he always delivers.
In fact, the entire cast delivers above expectations.
The plot is unique for Korean cinema, and I loved that there's no maudlin love story. The comedy works surprisingly well for such a grim story, too. The stereotypical Korean characters (screaming demon of a mother, unbearably stupid "comic relief" personages, open-mouth-eating-and-farting-middle-aged-men, and the like) are thankfully missing.
The only thing I found really frustrating was that no one asks the most obvious question. I mean, seriously, they start wondering about the most obvious thing 12 or 13 episodes in. That's the very first question I would have asked, and watching them not do this was infuriating! :)
I realize they needed it for the plot, but still.