The rate of failed withdrawal method has been quoted as anything from 5-30% when used correctly but the actual true amount is hard to find.
There is a myth that if a man "has great control" then you wont fall pregnant while to a small degree good control does help its not exactly how it works.
A man can not feel pre cum so if there is sperm present you have a chance of falling pregnant. Also orgasms vary in the same person and at times a man can not feel the very first bit, in order for withdrawal to have a higher percentage of success the man should pull out way before he is going to cum.
There has also been suggestion that withdrawal can lead to some sexual health problems such as erectile dysfunction and possibly cancer although this is not scientifically proven at this stage.
Bottom line is that withdrawal method is for those people who don't mind falling pregnant.
Like with anything some people will have success using it and really it comes down to luck. If there is no sperm present at anytime during s.ex then you wont fall pregnant but its like playing a bit of a game.
If you don't want a child use another method.