I think they help quite a bit during and after the run. They have virtually eliminated my calf pain during some of my longer runs. It had gotten to the point where I was almost unable to go long distances because of the aching. I would highly recommend them. I think Zensah is a good brand to go with. They're cheaper than 2XU or CWX, and work just as well.
I train with skin leggins and I use Cambridge compression leggins, I always got knee pain during a run and lactic acid after a long run, (anything over 5 mile). Since I bought these my pain or acid build up has virtually gone.
I bought some Skins Powersox just prior to running my first half marathon this past August. I find they have helped tremendously in stopping shin pain and lactic acid build up in my calves.