I can say this in all honesty, for me, once I came to terms with being poly, it has been as easy as breathing. For me, there are no new, additional downsides. Time management is my life struggle, as I work multiple jobs and will going back to school full time and working in the fall.
I feel completely content in my current situation (one husband, one serious boyfriend), but I'd also be willing to change. Here's the best analogy I can come up with. Imagine you had a job that you liked that paid you well for your work. Everything is pretty darn good. You could say that you were completely content with your job and could be happy with it for the rest of your working years. However, if someone offered you a better job, that paid better or was more interesting, you might take it, and find yourself in a new situation, where you could be complete content. I don't mean to say I'd dump everyone and move on, but rather that if more good could be had by adding someone else, it would be cool. I hope that made sense.