I am an only and have 3 dc's, as I always fantasised about a big noisy family. However, as I see the reality of the bigger family, I now realise there are up AND down-sides to being an only.
My parents never intended me to be an only, but my mum lost 4 babies after me (2 mc's, 2 stillbirths) so I lived a fair bit of my childhood waiting for a sibling, who sadly never materialised. Thankfully, my parents were very pragmatic and down-to-earth people- I was never treated as an only child, really.
I had a really close relationship with my parents, but was encouraged to go off and make friends my own age, especially on family holidays. We lived in a housing estate with loads of kids, and I had lots of cousins, so I wasn't really lonely. I do, however, remember crying when friends went home after sleepovers.
I'm very sociable- far more so than my DH, who has a big sister whom he hardly ever sees and isn't close to. As a child (very young!), I used to go up to other kids and say "Hello, my name is joolyjoolyjoo, and I will be your friend!" (Ok, bit of a freak, but I learned to take rejection as well as making a good few friends! I find it reasonably easy now to get chatting to strangers- in fact, DH is always moaning that I chat to everyone! On the other hand, I also learned to love my own company, to nurture my imagination and entertain myself. I don't NEED company, and am quite happy to amuse myself still, which has allowed me to be very independant.
The big down-side was when my mum got ill (she was ill for 20 years or so) and subsequently died. I did wish then that I had siblings to help with the hospital dashes and for emotional support. Dad and I are still very close- I actually moved next door to him after he had a mild stroke a few years ago. He sees the kids every day, is great mates with my DH and is very much part of our family, yet does his own thing most of the time.
The other thing I have been noticing recently is that I am limited in the things I can do with my kids due to the fact there are 3 of them, and they are varying ages. I feel I am always saying to my 5yo "We'll need to wait until ds is asleep to do that" and holidays are curtailed due to the practicalities of taking a wild 19mth old! When I was dd's age I had been to America/ Canada/ Africa/ France. I always thought I would have loved a sibling, but sometimes mine really don't seem to appreciate each other very much, to say the least!!
I guess what I'm trying to say is that it is swings and roundabouts, imo. I don't feel hard-done by as an only, but I did fantasise about a bigger family.