The treaty is not very effective. Those countries that want to get nuclear weapons generally can. Those who have tried and have not gotten them yet have not been stopped by the treaty. Instead, they have been stopped by other countries.
The NPT was not able to prevent India, Pakistan, and Israel from getting nuclear weapons. Of course, they did not sign the treaty, but they did get weapons, which is what the treaty is meant to prevent. North Korea has also gotten nuclear weapons. It has done so after withdrawing from the treaty. The treaty cannot prevent countries that haven't signed it from getting weapons. It cannot prevent countries from withdrawing and then getting weapons. This shows that it is not a very strong treaty.
The treaty is only as strong as the international will to enforce it. So far, that will has been lacking. It continues to be lacking with today's problems with Iran's nuclear program. The treaty is only enforced by countries such as Israel (as when the attacked Iraq's nuclear program), acting on their own to protect their own interests.
This evidence shows that the treaty is not strong enough to truly be effective.