My wife and I use NFP. This is actually the slickest app I've ever seen.
It's pretty reliable if you want to use it for birth control, provided the woman has at least somewhat regular cycles. Even with irregular cycles, there's useful information that can be gleaned from cervical mucus and basal body temperature.
It's probably not for people who absolutely cannot tolerate one or two "surprise" children, but it will at least prevent you from becoming the Duggars if that's not your thing.
