For an enjoyable and intimately personal read about Cars, check out my friend's comments on the movie (from his archived June 11, 2006 post) at Donutbuzz. I couldn't improve upon the sentiment, nor the writing.
For me, though, the film was wonderful for one, simple reason:
While watching Cars with my son Jaden (who, I've said before, is rarely still) the three-year-old snuggled up onto my shoulder and watched the movie with me. Calmly, and quietly.
For a short while, it was nothin' but magic.