Quoting 12.21.07¤04.18.13:" I laugh at people who say it works just because they haven't gotten pregnant within two or three years. ... [snip!] ... prevention, it's playing odds. I'm pretty sure standing up and letting the sperm run down your leg will "work" for awhile too."
It works pretty well because sperm is not put into the woman's body if done correctly. Every single person who has told me they got pregnant with the pull out later told me that it wasn't being used every time (4 separate people). Not saying people don't get pregnant while doing it, but I've read it has a 4% failure rate with perfect use (condoms are about 2%). One thing is some men cannot control when they start to ejaculate, so this can create a problem right there. I know at least 2 couples who used the pull out without getting pregnant for years, then got pregnant the first month they tried. One couple had 3 kids the first month every single time. I personally wouldn't use it unless I know for sure I had already ovulated a week before, if I wasn't TTC. 4% is too high when you're deadset on not getting pregnant.