I'm an only, and several of my close friends are onlies. None of us are unhappy, nor were we unhappy as children. We all get on really well with our parents, particularly as adults we have incredibly equal relationships, whereas other friends who have several siblings seem to be still stuck in that parent-child/baby of the family/ 'Daddy will fix it' rut. This may not always hold true, but honestly it's as true as all those helpful "lonely only" myths that so many love to perpetuate.
I do find that I am much more choosy about friends than others. I don't accept bad behaviour and just put up with it, and I am generally very happy in my own company. With that goes the fierce independence and occasional anti social tendencies. I socialise when I want to but I certainly don't feel obliged to.
I went on holiday alone once as it was something I desperately wanted to do and none of my friends were that interested. I have friends (multiple siblings) who were utterly horrified at that, and friends (only) who went on to do similar themselves.
I was not spoilt in a material sense as a child, but I was incredibly spoilt for time. My parents always had an abundance of time for me, sharing hobbies and interests, allowing me all the benefits of being an only. I have never, ever, felt in any way deprived by my lack of siblings, other than in a purely academic, speculative "what if" kind of way, the same way that I wonder how I would look with red hair, or whether I would have enjoyed being a teacher or a rally driver.
I genuinely believe that the positives outweigh the negatives, if they actually are negatives; I have often found that it is other people who are more scandalised and judgemental about my lack of siblings than I ever was. My parents were desperate for more children, as they are both from big families, so I think they felt it more than I did. They never let me feel like I was not enough though, and I think that is the very core of how content I was in my own skin as a child and as an adult.
Sorry for the long winded lecture. I have just been aware of being an odd little only child my whole life (in the eyes of others) so I have been well prepared to explain why I'm not, whilst simultaneously confirming their suspicions by refusing to bend to their idea of social pressure. 