I've noticed a lot of Japanese movies (those of Takashi Miike, Kiyoshi Kurosawa) tend to have a build-up throughout the movie where they constantly push your suspension of disbelief - to the point (in Doppelganger, for instance) where it changes from a dark drama to a comedy. Save The Green Planet uses similar devices, but in a much more complex fashion. At various points in the movie, it really does feel like a comedy, a horror movie, a bloody action flick, a conspiracy theory science fiction movie with aliens, a psychological thriller, a commentary on Korean society and prejudices.... It really does do all of these things: by putting into question the sanity of the lead character early on, and by validating or questioning different beliefs of his at different times (combined with clever flashbacks to tell you the background), this movie easily changes it atmosphere and genre impressions many times throughout the movie - to great effect.
It's also not a spoiler to say that the acting jobs are very good - Hwang Jeong-min puts in a very good job as somehow-disables girlfriend Su-ni, Shih Ha-kyun is great as the protagonist, Baek Yoon-sik plays the tortured CEO very well, and the inspector is quirky and hilarious.
It's also worth mentioning - and not much of a spoiler (if anything it could enhance the movie) - that this movie is clearly a commentary on many events in Korea (the specifics of which I'm unaware), as well as the treatment of the mentally or phyiscally disabled (a common theme in Korean movies apparently, if Sympathy For Mr Vengeance and If You Were Me are considered representative).