I have a friend who's tried it on two occasions. The first time, she and her best friend shared a boyfriend. It didn't last terribly long, but it seems to have ended amicably. The second time, she and her then-boyfriend decided to merge up with another couple into a foursome. Her original boyfriend eventually ran off with the new girl, leaving her and the other guy single(they weren't as into each other, and decided not to bother continuing after their other partners left).

Overall, she's mostly unhappy with her experimentation, and has said she intends to remain monogamous in future.