I saw this a couple of days ago and, as an enthusiast and scholar both of ancient history and comic books, I found it engrossing. Of course, all the characters are very two-dimensional and it's dificult to have much sympathy with any of them--but the story of the 300 Spartans is an enduring one in the West and probably most viewers already have at least a vague notion that the Spartans were Greek and ferocious warriors who saved Europe at the battle of Thermopylae (the Hot Gates) by defeating a far more numerous Persian army. As long as you know that much then you can sit back and simply enjoy the stunning graphics of a battle you will never be asked to take part in. The battle in the pass is gory enough to satisfy the blood lust of the most hardened young gamer (and many of them--between 12 and 15--were present in the cinema. Strangely, the movie is such a gore feast that the gamers seemed to expect all reference to a "real" story to be missing--and the 15 year old guys next to me chattered through the few extended sections of dialogue.
I enjoyed Frank Harris' changes to the true story--such as making the traitor a deformed Spartan rejected by Leonidas and portraying Xerxes as a bi-sexual giant. After all, this is Movie Land meets Comic Book World--and all things are possible.
This was an unusual and interesting adaptation. I advise you to go out and see it--if you haven't done so already!