Interesting concept, I wish more had been done with it.
It was a well directed film, although I'm not entirely sure why it's getting such widespread critical acclaim. I feel like I'm missing something.
I enjoyed the "tonal shift", which started framing the kid a the protagonist, and then shifted quickly to making him and his family the antagonists (parasites). I think that was the most interesting part. Unfortunately it happened one-quarter of the way in. Once the family had secured their positions, the film meandered quite a bit. Introducing the former housekeeping and her husband as bunker-dwelling parasites didn't really add anything for me.
I will mention that the characters' clumsiness nearly broke my suspension of disbelief, twice. The characters clumsily fall or drop something down the bunker stairs, each time significantly impacting the plot.
Overall, worth watching, not worth gushing over.