I love most things about being an only child, but I've seen my mum have to go through looking after her parents single-handedly as they got older (as she's an only child too) and that's the one thing I am worried about. It's not that I don't want to look after my parents when they're older- I'm happy to, since they looked after me for so long, and I love them very much- but it would be nice to at least have the emotional support from a sibling or two.
I've never been lonely as an only child, though. I guess I've been very lucky in my friendships and I also feel very independent and ready to 'take on the world', so to speak, so I'd see that as an advantage of being an only child. Depends on the parents, though, as many only children are mollycoddled and so they don't develop that independence, which is a shame.