Less than 99% effective, but that's largely because some of the time they are used incorrectly. If you used it right, you should not have gotten anyone pregnant. If someone is saying she is, you have every right to ask for a DNA test before believing it.
Getting pregnant is very difficult if you use the condom correctly... I think it's like 99% effective. But it's always safest to use a backup form of birth control if you're very concerned.