Open relationships should be no more or less healthy than any other, as long as there is consent and good reason. If one partner requires more than the other is willing or able to give (e.g. a particularly odd kink), or if you and your partner have a wonderful companionate relationship but are sexually incompatible, or if they just want to have fun, then there's no worry.
I'm partial (on paper) to Dan Savage's "monogamish" plan. You are monogamous, but there are certain, very specific scenarios where you can have something on the side.
I myself however could never do it. I'd be all insecure and jealous and it wouldn't work at all.
But Caedus, there's nothing intrinsically unhealthy about an open relationship, it's just not something that you (or I) could go for.
-Sparks Nevada, Marshal on Mars
"Who cares what evil lurks in the hearts of men!"
"Unless evil's carrying the Martini tray, darling."
-Frank and Sadie Doyle