They can work for some people, but not all people. It requires major research, commitment, soul searching and a strong understanding of who you are, who your partner is, what your goals are and what you mean to each other.
If the girl you are interested in is in an open relationship, you need to discuss these things with her so that you don't end up with a "difference in opinion" about the nature of your relationship. Sure, you may say "well, that's moving too fast!", but I find that people don't communicate enough and are not honest upfront, which just leads to trouble and heartache later.