I've been member of three MLM company's. Started once with Herbalife, joined DHS, quit both for several reasons, didn't do anything for a while and now I'm a proud and very happy partner of WIN Worldwide.
I can't agree more with the comments of others that being successfull with an MLM business has everything to do with your own capabilities. Most of all your capabilities to learn. Almost all MLM systems work, or they'll die soon.
But... having said that, I also strongly believe that part of your success is hidden in the used system and the help of your upline. I quit Herbalife mainly because my sponsor wasn't interested in showing me how to learn to work my business. Worse even, her sponsor didn't show interest either when I asked her to teach me... I don't believe Herbalife as an opportunity is bad, but I won't recommend it either. Because to my opinion, you can't rely on their support (maybe that's changed in the past few years, but still...)
If you're looking to start an MLM business, don't just look at how long they're on te market and whether or not you like the product(s), also look into both the payment system as the "work" system. What do you have to do to gain prospects for the business and the product(s). And most important, especially when it's your first MLM business, try to figure out how sincere your sponsor is to really show you what to do, how to make the system work. What kind of help does the business and your sponsor offer.
Going from a paid job or a "regular" business to MLM, is a big switch. You've got to give yourself some credits and time to fully understand the systems benefits and it's possibilities. Which does NOT mean that you can't make money with it, meanwhile. Just... don't quit because you didn't reach rank so-and-so within three months.