I was an only child and it is ok being one, but honestely, I always wanted to have siblings and I think they are important to a child on the long run. The big majority of my friends who have siblings have a close relationship to them and they are an important part of their live, especially in the cases where there were other problems in the family and siblings had to stick together.
But I guess everyone will tell you different points of views and personal experiences on this one