Here's a good start where you can read what is *wrong* with multi-level marketing:
Great Site
Also, there are other terms that MLMs such as Market America uses - "Network Marketing" -- same crap, just a different name.
Keep in mind that MLMs such as ScamWay can make plenty of money off the salespeople -- even if the salespeople don't sell products. ScamWay will convince you to buy their materials to make you a better salesperson and they'll recommend cassettes, videos, etc. -- which they'll sell to you at a good mark-up.
No, I've never been involved with an MLM. However, I do know plenty of **suckers** who have been drawn into them, only to suffer and fail miserably.