How effective is China's one child policy?

Officials claim that yes it has worked and it has prevented between 250-400 million births.

Whilst this may be so, the policy has been critisised by many who claim that, among other things, there have been forced abortions, abandonment of female children due to the traditional preference towards males and that it will cause long term economic problems due to a disproportionate ageing population (ie not enough younger people working to support the older generations).