I think that if you're old enough to take a fully informed decision (16? 18? 21? whatever is used usually in your country), then if you want to smoke, fine. Same goes with drinking and non addictive drugs (marijuana), or driving, or having sex.
As for the places, I'm for keeping closed public places smoke free (eg: libraries, museums, town hall...), for open public places (parks, streets...), as long as you don't blow the smoke right in my face and throw the cig butt in a trashcan/sewer gutters but not on the grass/pavement, it doesn't matter to me. Private places, it's up to the owner (though I like restaurants that have both sections).