We used it and it has never failed for us. What DID fail was our sticking to it ("just this once" is NOT something you should decide which actually in the act
Hello ds2) and deciding to reenter right after the fact (Hello Baby3!
: ) But when actually doing it correctly, it didn't fail. However, my dh pulled out before time. If your partner pulls out and is immediately *ahem* that is too late. He may have already done a little inside. My dh pulls out and a few seconds go by with "assistance" and then *ahem* happens (hoping you can understand this, I'm trying not to violate any UA). I believe that is when people fail, you can't pull out the second it is happening, it is already too late at this point.
Of course, this isn't much fun for dh or myself and because of this after this baby is born alive and healthy he'll be getting a vasectomy. Against much of what I believe but finally I just had to agree. He doesn't enjoy the deed as much because of pulling out all the time and he REALLY doesn't want another child (even 3 is pushing it for him). So there you go.
But, if your partner isn't opposed to it, and can catch himself in time and you aren't super fertile (if your partner can cough and you're pg, this is NOT for you) this is a great option.