We have had 4 newfoundlands and all of them were/are the most loving dog you could ever ask for. I don't know about how good they are for guard duty, but they'll be good for just about everything else. The one we have will, if she wants some attention, go up to you and lean on your legs until you scratch her right hip (yes it is at least 99% of the time she wants this hip scratched) to her satisfaction, by walking away after growling then "bwoof"ing at you everytime you stop or you fall over from her putting all her weight on you.
http://habee.hubpages.com/hub/Guard-Dogs-for-Families-with-Small-Children