I use them on a screened porch. Mildew has never been a problem, but fading has. Stay away from red if that might be an issue for you.

I live in northern Oklahoma and bought an outdoor rug at Walmart a few years ago. I used it for two years on a north facing porch that was covered. It got wet a lot and never mildewed that I noticed. My biggest complaints have more to do with the Oklahoma wind that kept blowing up the corners and the chairs that rusted on the rug.

I'd probably get another one if I found a pattern I liked. I'm also considering just painting one so I can enjoy it all year round.