If you get a chance to, take a look at the actual battle that took place. The Battle of Thermopylae is STILL taught as one of the great examples of a small, tactically-minded, well-trained force taking on a MUCH larger, but undisciplined force, and inflicting massive casualties before their inevitable loss. 300 takes a lot of liberties, but it's a very good entry into learning about one of the most influential battles of ancient history. Had the spartans and associated troops NOT held their lines, Xerxes would have had a much larger force with which to fight the Greek nations, and might have actually be able to defeat them, rather than being eventually force out of Greece. Also, it was a strong blow to the Persian's morale, who thought that they would walk all over the smaller greek army, and when it took them 3 or so days to take on a piddly 1500 or so, it definitely weakened their resolve to take on the full armies of Greece.
Sorry for the lecture, but the actual battle was NEARLY as interesting as the movie '300'.