I'm an only, and I've felt differently at different stages of my life.
When I was young, even though I had a very happy childhood, I would often beg my mum to have another baby because I was jealous of my friends who had siblings. Although, I do remember those friends at the time telling me how lucky I was not to have to share everything, including my mum & dad.
By the time I was a teenager, I'd changed my mind, and I was glad I was an only child. I had my lovely dad wrapped around my little finger, and I didn't want for anything. I'm sure it would've been very different with more children to take care of.
By the time I was a mature adult, I'd changed my mind yet again. My parents both became ill younger than I would have imagined, and I had no one to share the burden. The stress of taking care of them, first my mother, then my father, and the heartache of eventually losing them, without having a sibling to share all the ups and downs with, was extremely difficult. The sadness, loneliness and heartache that I felt almost broke me. I did have lots of support from my DH, and my own adult children, and other members of the family helped and supported as much as they could. However, I don't think that this was the same as having a close sibling to share these different life stages with. I basically had to put my life on hold for a very long time, and it's a lot for one person to cope with.
For this reason, if there's a choice in the matter, I wouldn't wish being an only child upon anyone.