It is an often-effective way for horndog guys to con gullible girls into having unprotected sex. If you mean how effective is it as a means of birth control, it is not effective.
I agree with the other poster. While ONE study showed that when done properly, the pull out method "only" has an 18% failure rate (that means 18 out of 100 times, it will not prevent pregnancy), it has ZERO effectiveness for preventing sexually transmitted diseases.
According to the CDC, 25% of teens have an STD (that means 1 in 4 of teens have an STD). Many do not even know it. Please, protect yourself and use a condom EVERY time.
It is NOT effective at all!!! DO NOT rely on this as your only method of birth control. Remember, it only takes one sperm to make a baby and there can be sperm in pre-seminal fluid.
Also, I would suggest using at least condoms since there is the chance of STD's as well.
Better to be safe than sorry. :) :) If you have any other questions, please ask. :)