And even when they aren't outright scammers, they often cause terrible trouble for you, as when they sign you up for the "super duper deal" with the local cable TV/internet/phone mafia, and then you really learn what frustration is, as they start billing you for things you never ordered and that the pleasant young sales person repeatedly assured you that you'd never be charged for. If there's a hell on earth, it might be in the wasteland of frustration and angst experienced while trying to get one of the big telecom companies to rectify a "mistake" (funny how the mistakes are always in their favor).
Bottom line: if you are interested in what they are selling, ask them to leave a brochure and a card, and then shut the door. Do not sign anything, no matter how hard they wheedle and whine. Take your time -- investigate the product or service on the internet. The deal will always be available, or a better one will. DO NOT let yourself be rushed "because we're only in this area today and this deal will be gone."