I used it too for a while and always in conjunction with OPKs. I would agree that consistency in its use is very important or you could end up quite confused. Same time each day and be very religious about it. It's possible to have ferning at a few different times during the cycle. It's also possible to ovulate and have no ferning whatsoever.

I think it works really great for some people. Those of us who have a few hormonal inconsistencies might have a problem with it though.

To what extent saliva correlates with cervical mucus, I am not sure. Both are bodily secretions obviously and can be affected by things such as antihistamines etc. which is worth remembering for people who have allergy problems.

In my case, I have not had much in the way of cervical mucus in recent years which probably contributed to my fertility problems in the first place.