It depends on what kind of soup. A light brothy soup I won't do for dinner, but they make good lunches. But something like my chicken tortilla soup - thick base, meaty, filled with veggies - I make for dinner all the time and no one ever complains about it not filling them up.
I'm with the group here. It depends what kind of soup it is. Thick creamy soups, or soups with a lot of stuff in them (like minestrone), make a great meal by themselves or maybe with a piece of bread. If it's a lighter soup, maybe a sandwich or something to go with it.