In fact, neither one can boast about their charming personalities. Ethan is a total goofball, while Peter is arrogant and uptight. By the end, Peter is going to develop warmer feelings towards Ethan, but I suspect it's not a lasting change. The movie's premise borrows heavily, if not entirely from John Hughes' "Planes, Trains and Automobiles", starring Steve Martin and John Candy. The only difference between the two movies is that Hughes' version had a heart and characters that grew on you, while Phillips' version tries too hard to make the two leads unlikable in order for the gross-out humor to work. At some point I had the feeling that the only reason why I was still watching the movie was just to see how much lower the characters could fall.
"Planes, Trains and Automobiles" was rated R, because of a single scene, where Steve Martin blows a fuse at a car rental office, resulting in the use of 18 F-bombs in exactly 60 seconds (IMDb.com -approved Trivia), resulting in a hilarious punchline. That gag had every right to be there, so the sacrifice of having the movie rated R was well worth it. But "Due Date" takes pride in its R rating a little too much. F-bombs are dropped every other minute, the story features drugs and drug dealers (Juliette Lewis), there is a scene where Galifianakis explicitly masturbates while his dog vigorously follows his example. Also, there are several scenes of wanton violence, like, for example, when Peter gets beaten to a pulp by an angry Iraq veteran in a wheelchair. The movie earns its R-rated stripes with sadistic glee.
"Due Date" has occasional bursts of wit, plenty of mindless fun and gags that work more often than not. The two leads, obnoxious as they may be, do a great job and make the dysfunctional duo work better than it should. All in all, if you like your comedy raunchy, this is a better than average example. If not, don't say I didn't warn you.