WHAT'S NOT TO LOVE??? That's usually my response.
I do have to clarify for a lot of people though that it's not just about the sex. (That's usually the first thing they think when they see me walk in with the wives)
It's a little bit of what everyone here has said. The communication that we all share is unparalleled by any monogamous couple we know. I'm not saying that Poly is the best way, but that's one of the most obvious perks to our lifestyle. I doubt that my love for my wife would be as deep and profound had we not started the poly lifestyle. Trust is NEVER an issue. How great is that??
We can all look at and flirt with whomever we want!! I never have to worry if I'm gonna get slapped because I looked at that girls...you get the idea.
I also never have to worry about jealous feelings because of that level of trust that we have.
All in all, I don't see any downsides to being in a Poly relationship. Sure, it's tougher at the start than a monogamous relationship, but as far as I'm concerned it's all worth it in the end.