As I understand it, Samurai armor for much of their history was constructed for protection mainly from arrows and not against blades. Karl Friday puts it best with this quote.

"Audiences schooled in samurai movies and television are sometimes startled to learn that mystique and symbolic value notwithstanding, swords were never a key battlefield armament in Japan. They were supplementary weapons analogous to the handguns and knives carried by modern soldiers."

This fact should not downplay Samurai armor's effectiveness against both arrows and blades however. Samurai armor itself was lamellar and was created from lacquered iron scales or laced leather strips. These materials were used to create rectangular scales that were adept at absorbing shock through diffusing the energy of blows through created layers. Friday characterizes the armor as offering better protection than chain mail.

On the effectiveness of the armor in battle, Friday states that "even expert swordsman under optimal conditions cannot readily cut through Japanese armor. Sword techniques developed during the late medieval and early modern periods for use against armored opponents target gaps and weak spots in the armor, but this requires considerable precision and skill, even when fighting on foot and wielding the sword with both hands."

All quotes and paraphrasing come from The First Samurai by Karl F. Friday.