As has been said multiple times, this is planes, trains and automobiles but less funny. It's really difficult to feel any connection or sympathy with either of the characters. Downey's character is a class jerk. Galfianakis' character was too quirky. I had to go home and watch the Hangover to get this dud's taste out of my mouth.
Easily on par with Old School, Wedding Crashers, etc. In some ways it even surpasses them. Being someone who goes to Vegas frequently, I can't help but think, 'there but for the grace of God.' Hysterical from start to finish.