Beavis and Butt-Head were pretty much the poster boys of a generation in the 90's. While people dismissed them as simply being crude and idiotic, people tend to forget there was an underlying social commentary in there, and it didn't take a genius to notice it.
The titular characters are by no means intelligent or popular. Butt-Head is at the bottom of the social totem pole and Beavis is just slightly below that. They see life from a very basic perspective: Something is either "Cool" or it "Sucks".
Their misadventures would often involve them trying to get girls or money (They only ever came close a few times with the former and were successful various times with the latter). These antics would be broken up by clips of the duo watching various music videos and providing commentary which could not only make or break record sales at the time, but also made them come across as "Idiot savants". Which brings me to my next point: The boys would oddly show little bursts of brilliance and wit from time to time (Albeit usually by accident) and that's where the satire lays. Like it or not YOU knew some dumb guys like this at some point in you're life, you may have even BEEN like that. Beavis and Butt-Head works on two levels: Either you got the joke or you were the joke. They were the animated embodiment of teenagers in the 90's and to some extent still are the embodiment of the teens of today (Which is sad to say).
In terms of it's content, parents need only be mildly cautious. The language they use is actually cute in it's own right. Calling someone "Dillweed" or "Buttmuch" is incredibly tame compared to South Park's Eric Cartman calling a group of his classmates "A bunch of a$$-ramming Uncle F**kers". Even when B&B mention genitals they only ever say "Thingies".
Violence is mainly slapstick....literally it's Butt-Head smacking Beavis for saying something stupid (There's irony for ya'). There are some episodes (Mainly from seasons 1 and 2) that are more over the top, but really, I would say seasons 1 and 2 are the only really unsuitable ones.....and the Halloween special from Season 6, but then again....it does its job and manages to be creepy. No more violent than a Halloween episode of The Simpsons.
For the parents, I will end this review with the shows very own disclaimer:
"Beavis and Butt-Head are not role models. They're not even human they're cartoons. Some of the things they do would cause a person to get hurt, expelled, arrested...possibly deported. To put it another way: Don't try this at home" :)