I travel to Mexico several times a year, by both air and personal auto (crossing the 20Km interior customs control post). I never get bothered, even when I've received the red light (yes, they make an inspection... but it's extremely cursory). I believe that I get somewhat of a free pass because I am a light skinned caucasian.

It always appears to be they are on the look out for people bringing in substantial gifts for Mexican family and friends... more than likely, these people are hispanic. The only people I ever see getting really hassled at Mexican customs are either Mexican nationals, or those who are Mexican in appearance.