Some John Hughes comedies are a bit lightweight, a bit frothy, a bit kiddy-friendly. They might make you smile, but they won't necessarily make you laugh out loud. This one's different. There are some real classic moments in it. The bit where Neal and Del wake up in bed together next morning ("Those aren't pillows!"). The bit where they are sharing the back of a truck with an alsatian dog, and it is so cold that their mouths have frozen in a permanent grin - including the dog. The bit where Neal is asleep in the car, and Del is singing/dancing/playing along to Mess Around on the radio instead of concentrating on driving. The bit where the traffic cop stops them, and Del tells him that even though his speedometer, his temperature and fuel gauge and all other instruments are knackered, his radio is as clear as a bell. And the bit where Neal vents his anger on the receptionist at the airport car rental place.
The last ten minutes of the movie, where Neal eventually gets home to his family, and we find out the truth about Del's wife (which the story has been leading up to all the way through), is really emotional. Much more emotional than you would expect from a John Hughes comedy anyway.
I love this film. I can watch it over and over again and not get tired of it. There's not many comedies where that is true - Spinal Tap, Life of Brian and Office Space are probably the only others. Del's witticisms (or lack of) get on Neal's nerves so much, that we laugh along with them, not because they are funny, but because they are so unfunny.
John Candy was a great comic talent who is sadly missed - some cruel people might say the same about Steve Martin after the crap stuff that he has done recently. This was made when both of them were at their peak.