Honestly, they're a pricey way of doing something that you can do yourself, and with women i've talked to they're average at best, most women prefer getting to know their cycles and cervical mucus with greater confidence. It's too complicated to describe in a single post, but your single most efficient ovulation predictor is to get to know your cervical mucus and the changes in it. The changes are quite varied and very distinct. You may also take your basal temperature to help narrow the date down, but that varies greatly if you don't wake up the same time every day.