I'm pretty thin, hovering around 45kg/100lbs at 162.6cm/5'4". I don't really have much in the way of boobs, and my face looks vaguely vampirish/undead, so I certainly wish for a more curvy body.
Getting to a healthy weight is a challenge, though - at least, for me. I get sick in the mornings if I eat, and I don't have much of an appetite in general... and when I do eat, I get full really fast. Weight gain shakes are helpful, but there are no miracles. If I want to gain weight, I will probably have to go beyond my stomach's comfort zone... and I might have to lose a few meals in order to find out just how much I can eat in a single day.
It would be nice to put on 20-30lbs. Am I putting forward the appropriate effort? No. I guess I don't think it's worth all the trouble and expense.