Saliva fertility?

I recently obtained a small microscope device that allows you to check your fertile days through monitoring your saliva. Th brand I got is called "know when", but I've seen many other brands. I'm just wondering if anyone on here has had any experience or success with these type devices? I never see anyone post about them and the packaging says it's 98% accurate. It seems to be much more cost effective than buying ovulation kits every month. Any info or success story welcome. I'm giving it a go!