I was in one for a while selling the product online... I didn't do the whole tell all your friends and family part of it, I.E heavy recruiting. Rather, I knew some people that sold the product and saw the success they were having just retailing online, so I gave it a shot.
Overall I was fairly successful, I turned a profit in the first year, but the majority of my experience was setting up any retail online store. I did game the recruiting benefits (without getting into all the technical details) but at no cost to those I personally signed up... I bought their stock and paid their signup fees for the most part as it benefited me to do so. On a good month I moved $30k of products so it was easy to play the game with that much stock coming in and out.
In the end, I'm glad I did it because I learned a crap ton ranging from how to get an online business going (none of which was taught to me by the MLM company btw) to how to have a positive attitude and stay focused on goals. Some of the reasons I quit doing it however are your typical reasons you might expect to avoid MLM altogether - the people are very greedy and toxic overall, and I also learned I did not enjoy selling & shipping product day in and day out. I found myself just waiting for that next notification that I made another sale... which could come in 3 minutes or sometimes a few hours... early on it could be a few days.
What ultimately got me out is that I signed up for a brand new MLM selling nearly identical product in anticipation to be higher up the chain when it got more popular. The first MLM caught me and quit selling me the original product I was selling very successfully, but in the end I was glad it was all over. I had roughly $8k in stock on hand when they cut me off so I thought I was screwed, but luckily I sold every last box of product I had.