Excited at first, but didn't end up working for me. To use the thermometer there are a few exclusion rules or else you can't count the temperature, including: If sick (colds happen often), Slept >2 hours more/less than usual (I am a student and during weekday I sleep in and so I can't count it), Woke up close to measuring time but went back to sleep, Woke up in night and didn't fall back asleep quick enough. Among other restrictions, it's just too much stress trying to recall if I can count that night or not. It was also annoying that it can't just automatically sync the temperature without having to go open the app and click the Bluetooth button on the thermometer. Not convenient, I have to constantly think about the birth control and all the hoops.
Overall takes far too much thought to make it a routine, and feels more like a chore that maybe works, maybe doesn't that day. I understand the appeal, and great for people who can find a way to integrate it into their life but for me it just wasn't it.
I would likely give it 5 stars if I had the wearable option as It does the exclusion for you and sync automatically. However, then I'd have to go purchase an Apple Watch new version. expensive.
UPDATE: Natural cycles now has a band and all I have to do is open the app for it to sync (can also sinc previous nights if I forgot or didn't have time). I no longer have to worry about things like sleeping in late because it takes the temperature multiple times in night. 3 stars to now 4 stars (not able to refund or cancel subscription period purchased if end up not liking it)