Research would suggest that yes, fly predators are effective in reducing fly populations. You must use them as directed, however. For example, if you have 15 horses, and you only buy enough for two horses, the predators will likely be unable to keep up. The companies that market these typically sell them based on how many horses you have.
You should also keep in mind that other livestock nearby may reduce the fly predators' effectiveness or may mean that you will need to purchase more predators. Flies can travel up to 5 miles, so in order for the predators to be effective, all livestock farms within a 5 mile radius should use the predators as well. You can help the effectiveness by using other strategies to reduce external parasite (fly) populations, such as keeping manure picked up, ensuring waterers are in good working condition (and not leaking), composting manure, and practicing good pasture management.
