I do it. My chickens are safer IN their yard with clipped wings than they ever would be if they flew out of it... which they can do with intact wing feathers.
Clipping wing feathers doesn't hurt. Only clip the ones that are intended to be clipped, and only clip them the recommended amount. If you clip the wrong feathers, or clip too much, it can be like snipping your fingernail while clipping your nails, or nipping your ear during a hair cut. If you do it right, its like clipping your nails or cutting your hair.
I always do both wings. Better (in my opinion) to not be able to get more than a few inches off the ground, than to be able to get a couple feet off the ground but not be able to control where you go and smack into something. Ouch.