This movie has been hyped way too much by film critics, socialists and various snobs. Sadly it can't quite live up to all the hype. It's simply not in the same league as earlier South-Korean masterpieces like "Oldboy"  and "I Saw the Devil" . Perhaps it's not such a big surprise. "The Host"  by the same director was also an interesting and weird movie with some nice ideas, but it certainly wasn't a perfect movie.
"Parasite" is about a poor family who cheats their way into a rich family, getting themselves hired as teachers for the family's 2 children and as driver and housekeeper too. Some of the movie is quite fun and entertaining, but particularly a certain "cellar incident" just crosses a line where it gets ridiculous instead of funny. The actors are just not skilled enough and overacting can take the fun out of anything. It's also a bit of a problem that it's very hard to feel any sympathy for the poor family, they really are just parasites, taking advantage of a nice rich family.
There's a bit of violence and blood in the movie, but not that much for a South-Korean movie. Perhaps they should have added more of this, because the few scenes with violence are some of the funniest parts of the movie.
Is it worth seeing? Yes, certainly. Just don't expect a masterpiece or the movie of the year.