Your enjoyment of this movie will undoubtedly depend on your existing familiarity with Korean filmmaking. While I'm certainly not an expert on the subject, I do believe many of the more negative reviews come from western watchers who are less accustomed to the style and feel of Korean films.
The film takes its time to build up and wind down, but I never felt like it dragged out too long or got boring. Even the setting - 70's Korea - is exciting to get a look at and not explored, I think, in other films very much. I have absolutely no idea how well this flick represents the real Lee Doo-sam, but all the key performances felt very fitting and well acted and the cinematography is nice and does the job.
I might add that I tend to analyze film pacing, and it hits all the usual turning points at the exact typical time marks.
What I think is most distinctively Asian in the filmmaking is the tone. It is a serious movie about several serious subjects with plenty of gritty scenes and emotion, but it's not afraid to let "wacky" characters act wacky. Some critics call this being tone-deaf or not making up its mind, but I don't think that is it at all. I consider it merely a refreshing cultural difference.
If you care about the setting or subject at all, watch it with an open mind and I think you'll like it enough to not feel like your time was wasted.