It is usually mild as a child - but is much more serious as an adult. If a pregnant woman gets chickenpox, it can result in birth defects if early in the pregancy - late in pregnancy and it can be life threatening to a newborn.
Also if you have had chickenpox, you have a good chance of getting shingles when you are older. Anyone who has had shingles will tell you it sucks - especially when you get it in your eye.
It's better just to get the vaccine and avoid the possibility of these things.