Hi Lavender, I have been using Natural Cycles for several months, but under a year. The app is good and if anything it is over-cautious. However, this method of contraception definitely takes two to work. It takes you to measure the temperature correctly every morning before you get up. And to be aware of any affecting factors like drinking the night before, going to bed too late/ oversleeping, feeling under the weather. The app will flag it up if the temperature is not what it would expect to see, but still it relies on the correct temperature for its predictions to be right. It is worth reading the medical research on the app, it has been proven to be exceptionally reliable if used correctly.
As with the pill, the reliability issues are due to 'life's stuff'. When you are not well, you keep relying on it out of habit and you must not. Your temperature will not be an indicator of anything in this case.
When you have had a rough night, your temperature is not useable. Travelling across timezones etc. Best thing is if there is any deviation from normal, not to rely on it, same as you must not with the pill. It says in case of illness, the temperature is not reliable for the duration of the illness plus 7 days after.
If you apply common sense and stay aware of any affecting factors, it is an excellent contraceptive option.
Another big thing, you will obviously be completely unprotected on the Red Days and will be at the peak of your fertility. So it takes dedication from your partner to take care during those times. Not surprisingly, you would appear the most attractive to him then and you would feel the horniest yourself, so this is where the app has been known to 'fail': people have not got the self-restraint and get carried away in the moment.
If you have to take the MAP, the temperature is not useable for the next 7 days either.
But having said all that, it has been FANTASTIC to have the peace of mind and to be able to let go completely when it is a Green Day without having this fear at the back of your mind. The H is happier than ever, as he gets to have proper sex with a woman who is into it (before, the pill just killed off any desire/feeling). My sex drive has picked up no end since I have been able to relax. More women should be aware of the Natural Cycles app, it is great :)
P.S. Worth a note that the Natural Cycles app has been officially certified in several European countries and it has got a CE Mark.