They can be pretty accurate if you chart temps and CM together- that will tell you when you're fertile as you approach O, and when you've O'd (and also when you're likely to get AF). I've just started charting myself after coming off BCP, and was so excited to see my first O temp peak, and then temp dip just before AF showed. Track your CM to know when to DTD approaching O. Can highly recommend the book Taking Charge of your Fertility, along with either Fertility Friend, or the website of TCOYF...