Polyamorous relationships are like any kind of relationship. As in they work if the people work and don't if the people don't work.
To the OP. You mention that a previous relationship started to go wrong because... "What I found with my last girlfriend is, the more time you spend next to a spouse, the more the little niggling facts that irritate you about them start to show."
Now, first, this happens to ALL romantic relationships. Every single one. Usually around the four year mark, which is when the infatuation stage of a relationship starts to fade. What it is not, is a signal that you are with the wrong partner, or that you need a new partner. As mentioned already, it happens to everyone. Its the sign that you have progressed to the point in your relationship where... well, to be perfectly frank, where you need to grow out of the kid stage as a couple and in to the adult stage as a couple.
There was actually an article about this exact thing in the February edition of Psychology Today, which I highly suggest you read. I won't go into any of the science details or psychology behind it, because, ya know, it's a forum. If anyone wants to ask I'll be glad to share though.
The bottom line is, polyamorous relationships can and in some cases should work. The same goes for monogamous relationships. It all depends on who you are, who your spouse(s) is/are, and what you are willing to put into a relationship etc.
Oh, and usually polyamorous relationships only work if everyone loves everyone. It isn't necessary, but it definitely helps to be bi.