My Sitka pants have kneepads in them already. I really like them, and they don't bother me too much while hiking around. They are especially nice in antelope country, where everywhere you kneel or crawl seems to have cactus.
I'll say they help. +1 for me on using them. Some guys that I hunted Antelope with in Montana on the great plains one year gave me a hard time seeing me bring them along. They sure help though when crawling on the ground sneaking up on critters. Picking prickly pear needles out of knees or smacking a rock in the knee cap does not feel all that good to me at least.
My Core4 Element pants have them built in, with the option to take them out when you don't want them. It took some getting used to, but now that I have wore them a little I don't even notice them. (other than when I am kneeling down and my knees don't hurt)
I wear them. I used to use foam ones and now am working off the ones built into my pants (sitka timberline). They saved my knee saturday when I slipped on a rock face and smacked my knee hard. My knee still hurts and I cant even imagine what would have happend if I wasnt wearing them. I like how they dont have straps around my leg for more airflow/less restriction.
I also use the knee pads that come with the c4e pants. I just used them on a wilderness Nevada archery hunt last week and they were great. I put them in for a stalk and just kept them in as they didn't bother me. Very light weight yet comfortable.