The Advantages:
Being an only child can be either good or bad, depending on how you perceive it and how you are brought up by your parents. The advantages could be that you get the undivided love and attention of your parents. They would dote on you more and provide for you more - in terms of getting you stuff, toys, etc. Obviously, being an only child means that your parents have only you to spend the money on and not have to share it among other siblings. So, you'd get more toys than normal, more money to spend than normal, more inheritance than normal, and of course more love from your parents than normal.
As an only child, you may also be spared the complications arising out of having an overbearing, unhelpful, competitive sibling. It is no fun having a sibling like this, who hoards the attention of your parents, who in the eyes of your parents can do no wrong and is doted upon to no end, often at your expense. They may well walk off with much of the inheritance by virtue of being the doted one in the family. As an only child, you would be free of any of these complications.
As an only child, you may grow up to be more independent and able to fend for yourself better, if your parents haven't spoiled you by tending to your each and every need. Not having an older sibling to help you every step of the way may in that sense be beneficial and make you capable of looking after yourself earlier in life. Therefore, being an only child can certainly have its advantages.
The Disadvantages:
As far as disadvantages go, the "Little Emperor Syndrome" comes to mind. For those unfamiliar with the term, "Little Emperor Syndrome" refers to the Chinese situation involving parents and their single child. Of course, all of you would be aware of China's one-child policy. Little Emperor Syndrome is an unintended consequence of that policy. This is a situation in which the parents lavish their love, attention, resources on this one child