A generic road trip movie with two terrible people who continuously make bad choices in a predictable plot rife with even more predictable "gags". Galifianakis portrays an even dumber and less funny version of the Hangover while Downey plays a less intelligent and less humorous version of Tony Stark.
Seriously, it isn't worth your time. If you want a funny road trip film then go watch 'Planes, Trains and Automobiles' instead. Doesn't matter if you've seen it before. You will still feel a lot happier than if you had wasted your life on this film.