We have good friends whose 10 y.o. daughter is an only child by their choice, and like Widdly is planning with her little one, she is totally involved in their life. However, she is totally not spoilt - she can be a drama queen, but that is more her personality (e.g a tiny paper cut is a huge gash which requires blood transfusions, lol). They take her to most things, she even got to go to the Robbie Williams concert when she was 9! She's part of their lives and she is quite happy entertaining herself in an adult environment, so when we all get together for dinner, she will bring along activity stuff to keep her self amused, and she can sit for hours and do craft stuff. She still joins in with the conversation and wants to be a part of what is going on, but she isn't all me, me, me like you see with some kids - her manners are lovely. In fact, I hope our little one and any siblings we decide to have are as nice a child as she is, and I've seen children who have siblings who are much more spoilt and would not want my children turning out like them.
It all depends on how you bring them up as to how they turn out, not whether they are only children or have a horde siblings!