I am a textbook only child: the good, the bad and the ugly. It is the most defining factor in who I am. I was spoiled (tantrums, the whole thing), intolerant (as described in this article) but also very responsible, pretty witty and entertaining as a kid because I was surrounded by a lot of adults and the center of attention. My parents meant well when they tried to make everything perfect, but that's not how the rest of the world works. 

The thing is, I have dated only children and have close friends who are only children and they are not like me at all because they had parents who were far more detached and/or less indulgent than mine. Truly, the least selfish man I ever dated was an only child. But none of the people I know (myself included) who are only children are from divorced parents, so I'm not sure how that would affect things. For the record, I loved being an only child, but if I were raising one I would be more insistent that the child try different things, experience failure and take risks. I was way too cautious and never experienced failure until early adulthood and at that point I was incapable of bouncing back from it the way you should. But I think that's just good advice for any kid, regardless of birth order.

My son was raised as an only child. So was my dad.  My dad's elder sister passed when he was 2yo.  My son has three younger half sibs by two other baby mamas in his SpermClan but saw them only during periodic long distance visitation.  Both my kid and my dad are scary smart, funny, and creative problem solvers.  I am having difficulty dredging up any memory reference to them sharing though.

IMHO the greatest factor is parenting.  Parents who raise children with behavioral and performance expectations and hold their kids to performing to those standards raise well behaved children who perform to expectation.