I'm an only and I suspect it - or rather our family dynamic - was really good for me at some points in my life, and really shit at others. No cousins either, or family friends with children.
So when I was little I think my parents were great with me and I had lots of positive attention - mum taught me to read very young, did lots of stuff like den building and potato printing and that kind of thing. All the stuff I feel guilty for not having time to do with my two preschoolers now. When I was in primary school I had my own friends, and also neighbourhood kids I could knock about with.
But as I got older it started getting more difficult. By around 9 I wanted my own interests and they weren't interested in encouraging them, apart from one lesson a week. Weekends were spent with me in my room a lot, reading or playing by myself. Family holidays were spent on static caravan parks in bits of countryside where my mum and dad could pursue their hobby - I always liked the bigger parks best as there would be other kids to play with, my parents always liked the most remote sites with just one caravan in a field. One year we went to Cornwall and I body boarded every day, it was my best holiday ever. My parents hated it because even though the beaches were beautiful they weren't so great for the particular thing they wanted to do there. We never went back.
We hardly ever went to a theme park or family attraction, and I never had anyone to play with at them anyway. Things like a trip to an ice rink, cinema, or pool with flumes happened with friends and their parents, not mine. I look now at my distinctly unbiddable children and I can't imagine making them just fall in with stuff that they hated doing in all their free time, or sticking them in a corner with a book. Our free time now is primarily about making fun for them, and DH and I enjoy it along the way.
By my mid teens I had made some close friends at school and basically spent every available weekend at their house whenever homework allowed. During my 20s I had very little regular involvement from my parents. Now I'm a parent they're more involved in my life again, and as we spend more time together I'm starting to notice it becoming difficult again. But I know if I wasn't an only I wouldn't have had the education I had (it was a struggle for one, would've been impossible for two), wouldn't have gone the uni I did, and my life would likely be very different.
If I had only has one child I wouldn't necessarily think it a bad thing, but I would work very hard on making some big changes from my childhood. As it happens the decision was made for us!