As long as you get your shots on time, this method is very effective. With perfect use the shot is more than 99% effective; with typical use, the it's closer to 94%.
In other words:
Of those women who use the shot exactly as directed, fewer than 1 in 100 will experience an accidental pregnancy during the first year of using this method.
Of those women who do not use the shot exactly as directed, fewer than 6 in 100 will experience an accidental pregnancy during the first year of using this method.