I am an only child, and I am the mother of an only child, ds9. I was very close to my mom, and since she has passed I'm still quite close to my dad. I was always mature for my age, and I knew how to behave around adults. DS has an advanced vocabulary, because we are his primary conversation partners.
I am adopted, so clearly my parents chose to have one child. My mom was the oldest of 12, and she had already "raised" a brood. She wanted to be able to have a relationship that she didn't -- she wasn't very close to her parents (although I think that's more a reflection of them as individuals than the number of children).
Anyway, I thought I wanted 3 kids, but with waiting to establish ourselves, moving cross country, etc., we find ourselves with one child. It is true, he absolutely has advantages he would not if he had siblings. He goes to private school, we go on great vacations, we are able to afford a good amount of house for our area -- we just couldn't do this if we we're taking care of two. His college fund would also probably be half of what it is.
He is very happy as an only, he realizes he gets alot more attention, as well as the material goods.
So, those are the reasons it works for us. It really is a very personal decision. I would not tell someone to only have one child if their heart is set on more, but if one works for your family, there is nothing wrong with that.
Oh! As an only who has lost a parent -- I always thought I had to get married (when I was about 10!), so I would have someone to help me when my parents passed. Well, I ended up being without dh for about 4 days when my mom passed, and I made it. I don't subscribe to the theory that you need to have more than one child to take care of you or each other. DH and his two sisters have absolutely nothing in common and are not close in any way, so having those siblings has not been a benefit for him.
I work in a department of 3 people, all only children. We are all very independent, self-starters, and we function well together because we have that self-reliance. We also understand that sometimes we just need our own space.
I get great peace from alone time, and I'm never bored. I learned at an early age to entertain myself. It's not all sunshine and roses -- of course I had to learn to compromise, and share. If you are going to have an only, just make sure they get plenty of opportunity for socialization, and I'm sure you will raise a happy, well-adjusted child.