Question for those using Fertility Awareness Method

I am very aware that many women on this site have used the Fertility Awareness Method (described in Taking Charge of Your Fertility) to chart their monthly cycles and conceive a child.
But, I am curious...is anyone using the same method as an effective form of birth control???

my SIL does, though not very effectively. 2 of her 3 children were concieved when she screwed up which was a good day for what