How accurate or OPK's?

So I've been TTC since April. Since we haven't gotten pregnant yet, I went out and bought an opk. Has anyone been successful with one? I'm so frustrated because when I wasn't trying, I got pregnant, unfortunately had a miscarriage. Now that we are trying, I feel like it's taking forever. Does anyone have any advice? I'm currently taking prenatals and Geritol. Any advice would be greatly appreciated!