ACN is a telecommunications company that provides services such as internet, long distance and local phone calls, and digital phone services. There doesn't seem to be anything substantially wrong with their products, but the company structure is designed as multi level marketing organization; or in other words, a legalized pyramid scheme.
As an ACN representative one is encouraged to recruit family and friends over cold calling or selling, which is a marketing scheme unlike other businesses. Additionally one is required to pay a renewal fee every year, and $10+ per month in other fees, as well as a fee for the marketing kit and the payment of any bills that aren't paid by customers that they've signed up.
Businesses like this are designed to make more money from their employees than actual customers. It isn't illegal, but it's a ethically questionable and an unsound business decision for would-be sales representatives.
I worked for a guy once who had a Shaklee distributorship, which is much the same kind of marketing. He was very upfront that one couldn't make money with it unless, as Spaw says, you get sup-contractors who have sub-contractors...etc. His pitch was that being a distributor got you good discounts.