I have never found them to be useful. Pull out your 16oz ball peen hammer, and give a good whack or two to the holding (female) side of a tapered joint, and they usually separate.
Some have used a metal bodywork dolly held to one side while applying said hammer.
I don't think they do damage to your parts, potholes are probably more harmful. I have missed a few times holding that fork and hit my hand.
