I don't regret it, no. DS happens to be an extremely highly strung, sensitive boy and has now been found to have SEN as well (dyspraxia) and I am 100% sure that he has thrived better with our full attention; he soaks it up and needs it far more than some other, more even-keel children, probably would.
For me, I regret not getting to 'do' labour and breastfeeding again so that I could 'get them right this time!'. but that is selfish reasons, for me, nothing to do with welfare of ds or putative second child!
I don't deny that i have wobbles, huge ones, about this issue. I have a close sibling and i fully know the lovely parts of having one so I do know that ds is possible missing out on one of life's joys; and as a parent you want your child to have ALL of life's joys, unrealistic though that may be. I wonder about ds' life when DH and I are dead and of course I sometimes allow myself to picture a lonely person with no immediate family.
However that is not something I can do anything about, other than to ensure ds sees all his family inc cousins, which he does, and that we help him value friendships and get good at making them, which we do.
So taken as a whole I do not regret ds being a singleton. For him, and for us, there are HUGE pluses to that situation. And will continue to be so, all through his upbringing I am sure; though these advantages are not all about money, Custardo's post about the reality of the cost of children up to adulthood is very important.