My husband was raised as an only child pretty much(his brothers are 11 and 13 years older than him) and he is awesome, totally well adjusted and a great guy.
He does get annoyed more quickly than I do when the kids bicker-but he has no clue what it's like to have someone so close to your age with you all the time(mine are a little less than 2 years apart).
I am the middle of 3 (brothers 3 years older and 2 years younger)and I know it can be annoying, even when you're older they can be annoying, LOL-but it's worth it because it's it's pretty great to have someone to turn to and say hey remember that time mom did.. or remember when we went on vacation to... or remember that Christmas when I was 5 and got...or remember that time you wiped out on your sled..or remember the Halloween that crazy lady yelled at us for ringing her bell..
That's what my husband says he misses the most. He has no one who knows what it was like to grow up in his house the same time he did with his parents. He never wanted to have just one child. But really, he's probably more well adjusted than I am.
My niece is an only child and is very good friends with my DD, they see each other a lot. Does she have some "only" stereotypes like a little spoiled and a little too used to having her own way and speaking to me like an adult instead of an 11 year old? Yes. But who cares? I don't have to spoil her or let her have her way when she's at my house, LOL. She's a great kid.
I wasn't an only child, but I was the only girl, so I was a little spoiled, and a little used to getting my own way-and I had my own room while my brothers shared so I was a little bit used to having my own space and really liking it. So you can have a non-only and still run into the same stuff.
I guess what I'm saying is there are pluses and minuses to both and you do what is right for your family.
When my DD whines that her cousin got this or that, I tell her I got her a brother instead and he's the gift that keeps on giving.
I think onlies often tend to be more outgoing too. I think that's a great thing. My BFF is 11 years older than her only sibling and she never met a stranger in her life. I have another friend who is an only and she doesn't have a shy bone in her body. Just from what i have observed from the people I know as onlies or almost onlies.
So have an only and enjoy every minute!