I believe it's a very funny film and I'm sure it can be rightfully called a comedy, but don't expect punchlines, gags, jokes or slapstick - it's not that kind of comedy. If you want to enjoy this film, you have to meet Jeffery "The Dude" Lebowski by his terms, hang out with him and his bowling buddies and follow them at a their (perhaps somewhat leisurely) pace through this weird and unbelievable tale about nihilism, theft (of a car and, more importantly: a Creedence Clearwater Revival tape), kidnapping, abstract art, porn and - of course - bowling.
The story is actually kind of simple. You see, some no-good German nihilists urinated on The Dude's rug - and this kind of aggression just won't stand against The Dude. With the help of his bowling buddies, he will do everything in his power to get someone to pay for his rug - or possibly get a new one (because that rug really tied the room together). So he embarks on this worthy quest during which he will encounter many wondrous things and fascinating people (even Jesus - who is NOT the messiah but a very naughty man).
With Jeff Bridges in the leading role, the Coen brothers have found the perfect actor to incorporate one of the most iconic characters that has ever been created. But it's not just The Dude that makes this a winner; the whole film is such an inspired folly and simply inventive filmmaking at its finest (the hilarious dream sequence alone is worth the price of admission). The supporting cast is outstanding (J.Turturro, J.Goodman and S.Buscemi among many others), the song choices are perfect and the dialog is the funniest, most quotable in any comedy I've ever seen. This - for me - is without a doubt the film with the highest rewatchability factor of all time. 10 stars out of 10.
Favorite Films: http://www.imdb.com/list/ls054200841/
Lesser-known Masterpieces: http://www.imdb.com/list/ls070242495/
Favorite Low-Budget and B-movies: http://www.imdb.com/list/ls054808375/