We are very happy with our DD (3.5) at the moment and feel no urge for another. I think we could give her a lovely life, still have time for ourselves and careers if we just stick with having one. I have gone back to work after 3 years and feel like a new person. The thought of going back to the baby stage fills me with dread. DD is gorgeous and bright but still not a brilliant sleeper but we can cope now as she's so much easier in the day and at full time nursery.
The only thing that worries me is are there any parents of onlies out there who regret their decision once their children have grown up ? I don't want to be a little old lady thinking "if only..." But right now there is nothing really pulling me towards having another child other than "as a sibling" which I don't feel is right.