It's unique having an only child.  Here's some 'positves' and 'negatives'  (for want of better words)... with a bit of humour thrown in, too!

POSITIVES: 

* not too many arguments about what movie to see, or where to go on holidays, or where to have a

meal (if we're out)

* often gets things new, i.e. clothes, toys, school things, sports equipment

* don't have to share treats because it's not a case of, "Now give the same amount to your sister!"   (We have to be careful here, i.e. we don't want to overindulge our son, so my husband and I often think, "Now would we do this if we had three or four children?")

* can give our son uninterrupted one-to-one time, e.g. help with homework, working on a school assignment/project, or jigsaw, so there's a greater chance of them doing well at school (true for our son)

* parent-teacher evenings don't drag out because I don't have to wait around for other siblings' teachers, too! 

* It's only pocket money for one child, not multiples of $5 notes, so we save money!

* the house is always tidy!  And usually quiet, too!

* you may have a closer relationship with your child because of the extra time you're able to have with them, i.e. don't have to divide your time between siblings

NEGATIVES:

* there is more chance they'll be lonely, because they don't have siblings, however in our experience our son's loneliness is minimised because we give him lots of social opportunities, i.e. Sports, music, other social opportunities.

* my husband and I are, at times playmates for our son, if there aren't other kids around for him to play with, i.e I remember my husband actualy 'playing like a kid' with our son, and our son was having the time of his life!

* in school holidays we have to make an effort to not spend too much time at home, or days could go by with our son not having a lot of interaction with other children

* when our son began pre-school, and later primary school he had a few 'social' difficulties, i.e. he'd happily seek to quite happily play on his own (and have to be encouraged to interact with other children) or he'd become overwhelmed with there being so many other children around

* it took our son longer to learn skills such as sharing, and waiting, because he wasn't in those situations very frequently, compared to children who had siblings.

* we'd have loved our son to grow up with brothers and sisters, as my husband and I both came from families with siblings

So that's a start!  If I think of any more, I'll add it!