How significant is emergency contraception?

I was freshly curious how effective emergency contraception is? Particularly Plan B, seeing as how that's the just one that I know of that can be bought over the counter. Has it worked for anyone? Has if failed for anyone? Plan B's own literature say it is "up to 89% effective". And that it works for "seven out of eight women".
http://www.go2planb.com/ForConsumers/Ind...
Not that terrific, in other words. For every seven joyous girls, there's one really unhappy one.
it have worked for me. ive taken it 3 times with int 72hours.