First, I'll state the obvious: no one can adequately review this
without comparing it unfavorably to Planes, Trains & Automobiles.
There are only a handful of moments when we glimpse how entertaining
it could have been (including at the very end, thanks to a popular
sitcom) had it been done as a romp, not a slog. Both Downey, Jr. and
Galifianakis work hard at their characters — Downey too hard: he
makes his unpleasant, joyless asshole of a character so vivid, it
turns the movie itself into an unpleasant, joyless journey. And like
the interminable road trip from Atlanta to L.A., the tone here is all
over the map — wacky at times, but just as often discomfiting,
including a truly ugly, wildly inappropriate, ick-factor physical
attack on a young boy (by Downey) and another less outrageous but
still nasty affront to Zach's dog (also by Downey). That a superstar
of Jamie Foxx's caliber would show up in such a nothing part must
speak to a favor owed. Only Juliette Lewis among the lesser players
makes you sit up and take notice, and she's not given much to work
with. Here comes my tag line: DUE DATE is doo-doo. — Jeff Schultz

Due Date may not be the prettiest newborn on the block, but it isn’t a stillborn as many mainstream critics are claiming. Downey plays a dick, Galifianakis plays a dweeb, both start off very annoying and end up not as annoying. When it’s funny, Date is very funny, when it’s not, it’s amusing. A note to directors: there’s nothing in the comedy rulebook that says you have to put in a car chase! Probably the only lame part of the flick, though not a dealbreaker. Bottom line: Funnier than Dinner For Schmucks and The Other Guys, not as funny as The Hangover, but totally worth the price of admission. And once again, refreshing for a comedy to use the “R” rating and go for it…some comedies SHOULD be for adults only! 
-Storm Curry