I agree with other commenters that this film has a serious tone problem, shifting from gross-out comedy to serious drama. Additionally, the way the writer has chosen to structure the story doesn't help. For most of the film, we're not sure who we're rooting for, or whom the movie is about. Is it Wade, the germophobic travelling salesman? Is it Pat the wandering mechanic who seems to have great luck in finding stranded motorists? Is it Pa and Momma and their redneck family? Or is it the trying-to-make-it-big rock band with one of the most unlikeable lead singers ever? The structural problems are a shame, because the actors have a blast with the material, especially the "redneck" characters -- who turn out to be much more than surface stereotype by the end. And if you stick it out to the end, some nice things and touching moments happen. Unfortunately, the last half of the film is not served at all by the first half. While several of those scenes do belong to Wade, the film should have either been only about his character and the southern family, or should have dropped him entirely.
Possibly, the filmmakers had Robert Altman pretensions but Porky's sensibilities. (For the reverse effect, see Robert Altman's O.C. and Stiggs.) The two don't mesh. If you run across this film on the $2 video table, then it's worth grabbing for one of those nights when you're bored. Like I said, the destination is nice. It's just a rough and uneven trip getting there.