How reliable are birth control pills?

No, because fools are so ingenious. Taken properly, Birth Control pills are about the most reliable form of birth control other than abstinence or surgical sterilization. However, the user is required to take them properly as opposed to, say, shoving them in her ear, or more likely forgetting to take one or more over the course of a month. Also, even when taken properly, there is still a small rate of failure.