Observations

Rating: 7/10 (22 August 2019)

Firstly, movie hype of the wildfire sort (ovation, talk of Cannes, etc) grossly overinflates expectations for a film. If you've read 50 reviews calling something 'best film ever' and then watch said film, I can almost guarantee you will not share their opinion afterwards. No, it's not a flawless masterpiece that will be remembered for centuries - but just because others think so is no reason to hate on it either, since it is undoubtedly an accomplished and entertaining piece of filmmaking.
Haters may hate for ideological reasons: something is definitely fishy with the skewed helpful/unhelpful ratio on most of these reviews (it's 1:5 to 1:10 for most reviews), which may or may not point to some sort campaign of orchestrated manipulation, or at the very least an outpouring of emotion. Clearly this has people roiled up!
Which is funny, because it's not really all that clear what there is to get roiled up about! Lots of reviewers talk about the underlying themes and motifs and whatnot, vaguely alluding to what those are - class struggle, a dash of political allegory - but no one so far has written up any sort of concrete analysis that decodes the film, pointing out the literary, cinematic, political, or psychological allusions in specific scenes. If you read enough of these reviews, you know there's always at least one arch-geek who takes on the challenge and posts something that borders on coherent and insightful. The North Korea references would be a great place to start, perhaps.
But yeah, I suspect this aggravated the sensibilities of some anti-anti-capitalist types, who rail against any lefty do-gooder subtext and want their films all storytelling and no agenda. (This is a silly expectation, since most storytelling throughout history is overtly agenda-driven.) Though it's definitely not a 'pro-capitalism' film, it is quite ambiguous about its morality, and there is no one to really root for or hate. This is actually one of its more subtle but grandiose achievements.
More to the point are criticisms of the acting, which may or may not be annoying (I don't speak Korean, so I cannot fully judge). Plot is on the improbable side, although the 'fun factor' of watching this unravel counterweighs the contrived nature of the initial setup. The final stretch doesn't quite deliver (if it did, this would be a 9-10), and there's an unnecessary and rather maudlin coda.
But overall, strong work, and definitely one of the year's must-see films.