They absolutely do help (I'm one of those contractors), but I'd opt for something with a flat profile: the more bulbous-fronted ones I use for kneeling on hard surfaces tend to leave gurt big holes in your lawn and flowerbeds.
Also, I hate to say it but those rectangular pads with sticky-up handles do work, to take some weight off your knees when getting up from the floor. It's the getting up from deep flexion which causes a lot of the pain and damage.
Are you taking omega supplements as well?