another only child of an only child here, with (probably) an only child. It is a different experience, not necessarily better (or worse). It should be remembered too, that in the past parents usually limited their families for economic reasons (half the kids in my mum's class in the early 1940s were onlies, from working class/lower middle class homes) and that ecologically there's no doubt that it's a good thing. If you grub about search on here, you will find plenty of examples of toxic sibling relationships, mostly rooted in childhood. My DP has 5 siblings, 2 of whom are great, but 3 of whom are troubled/problematic in various ways and are sources of conflict and sorrow.
On the whole, the only children I know (I include myself of course ) are strongly motivated, successful, resilient, sociable, have a powerful social conscience (aka a sense of themselves as contributors to society etc), cheerful and independent. If the goal of raising children is to produce morally and socially responsible human beings who can look after themselves (and others), then most onlies I know are successful and have been well parented. This really can't be said of many children of larger families, where there is always someone to fall back on.
Riven is right about the aging parent thing (we insist that our DD waves us off as we slug our pills and whisky, but I know full well that if we manage to bring her up at all well, that she will feel responsible for us as we dribble and wee). Being an only is also not great if things go very very wrong - my father died when I was 8 and that made me feel completely responsible for my mother from then on, which really wasn't healthy. Mind you, a different mother would have tried to have prevented that happening