While critics would be quick to point out a battle droid's mindlessness as a fault, the proponents of the mechanized soldiers would counter that it is instead a virtue. A mindless soldier is a loyal soldier, unquestioning of orders, easily controlled, and absolutely fearless.
It was the invasion of Naboo by the Trade Federation that showed the galaxy not only the effectiveness of battle droids, but also the glaring oversights in their design. Against a peaceful populace, the Trade Federation army of battle droids was able to quickly conquer and take control of the Naboo population centers. These droids would blindly obey orders spoken to them by their commanders or transmitted to them from an orbital Droid Control ship. The efforts of Bravo Squadron, and Anakin Skywalker in particular, destroyed the Droid Control Ship, thereby rendering the army useless.
Battle droids are tall, gaunt humanoids with exposed joints and bone-white metal finishes that gives them an eerie resemblance to animated skeletons. Battle droid infantry is frightening in its uniformity -- only a numerical marking on the back of a comlink booster pack serves to distinguish one droid from another.
Droids with specialized functions have distinct colored markings on their armor. Blue denotes pilot droids. Red denotes security droids. Yellow denotes command droids, which function with increased autonomy compared to the standard infantry.
Battle droids are very efficient. Their humanoid builds allow them to operate machinery and pilot a wide variety of war craft designed for organic pilots. For the Trade Federation, the battle droid infantry piloted STAPs, MTTs, and AATs as well as their colossal battleships. For ease of storage and transportation, battle droids could compress into less than half their size.
Following the Naboo debacle, the Galactic Republic enacted strict legislation that prohibited the use of battle droid armies. The Trade Federation has shown its disrespect for Republic law in the past, and for years, many feared that the numerous foundries scattered throughout the galaxy were still churning out assembly lines of tireless soldiers.