I'm not going to analyze the story deeply nor will I give any spoilers so maybe if you're just trying to figure out whether to watch it or not, hope this helps.
I don't think the actual concept of the story is anything original but I'm really suprised at how unpredictable they were able to make it. The time flew by and I didn't feel that there were any dull moments in the plot. Very well written overall.
Acting was great and the cinematography was amazing. I've watched some works with Park Shin Hye and I really liked Jeon Jong Seo in Burning, so I expected good acting and I wasn't disappointed.


*This might be a spoiler, maybe not.*
Even though I didn't really like the ending, that won't affect my rating because I don't think a happy or satisfying ending has to always be the goal in movies. This whole movie made me feel pretty unsettled and anxious but in a good way if that makes sense? (my english vocab isn't the best but I'm trying to think of words to describe my feelings, please understand hehe)
If you like open endings and psychological stuff, this will suit you really well.
I finished watching it maybe 15 minutes ago and I think I need a moment to do some thinking.
In short: Not for people who don't bother to think after or aren't capable of understanding "deep" stories in the first place.