I've used the rhythm method (combined with a diaphragm on fertile days) for twenty years, and I've batted 100% (non-pregnancy). This is not because I am not fertile: when I wanted to have kids, I did.
However, I should mention one big caveat: they have done studies on male/female affinity and its effect on ovulation and they found that women's ovulatory cycles can be severely disrupted if they happen to be with a man that they are extremely pheromonically attracted to. In other words, if the underwear of the guy you happen to be sleeping with smells like a spring meadow to you, use your birth control method every day:)
Sorry I can't cite chapter and verse on the studies. I heard about it on a long talk show on public radio a few years ago.
Joanna