I'm not gonna knock the products because there really are some good things that some of these MLM sell.
Rather let's target the actual system itself:
They are pyramid systems, over here we call them "the mouse's children" for a more European phrasing ... rabbit children.
Sure Amway is now legal and what not but still those at the top make the money. Those who join really late in the game.
Basically, a MLM is a timing thing. Get in early -- make money ... Johnny come late = SOL.
However, there is one thing that could be beneficial to anybody, especially those who are timid. Some of these system provide pretty good sales education, they get people motivated, they teach how to sell and what not. This alone sometimes is worth what you have invested.
So check out their education system and walk away a more motivated person
Then go off and build your own system
ahem ... can anybody say, internet affiliate