300 is not a historically accurate depiction of the Battle of Thermopylae--certainly everyone has realized that by now. This film is extremely stylistic, with a heavy use of green screen to simulate dramatic lighting and employ heavy amounts of CGI (there are plenty of ridiculous CGI blood splatters that simply vanish into thin air). Everyone in the film, even the Spartan politicians, is about as physically toned as is possible for a human, and the Spartans forsake battle armor to show off their hairless sculpted torsos.
300, based on Frank Miller's graphic novel, is kind of ridiculous. It's completely over-the-top. It seems like an homage to masculinity. And yet, as much as I want to laugh at it and dismiss it as stupid fun (which is easily done with the sequel, Rise of an Empire), this is actually an excellent film with some amount of depth to it.
No, King Leonidas probably didn't fight a giant CGI wolf as a boy, but this film is extremely successful in conveying the mindset of the ancients--their perceptions of an enemy, their need for honor and glory, their dedication to their nations, their struggle to understand the world around them, and their devotion to camaraderie and brotherhood. As the film portrays, this ancient mindset was often forged in conjunction with death, battle, and sacrifice. The style used in this film helps, rather than hinders, the conveyance of an ancient Spartan worldview.
That the movie managed to convey these themes while being entertaining, quotable, heavily stylized, and overall cheesily awesome is what makes it a success.