I think the fact that you have not read the novel destroys all credibility of your opinion.
Frank Miller's work is about as far from 'Hollywoodised' as you can get in terms of graphic novels being made into films. Robert Rodriguez had to quit the Director's Guild in order to keep Sin City true to the novel.
Nothing was 'adapted', the story is biased to Miller's point of view, that is his right as the author. Nowhere is it claimed that it is completely historically accurate - there are historical readings recommended on the final page, but of course, you didn't even get past the first.
The art work is beautiful and some of the famous quotes are accurate according to Herodotus. All Miller has done is blur the line between fantasy and reality, which surely is the point of a graphic novel and a film - it is not a documentary, nor is it a historical account, it is a story. It may be biased, giving a distorted view of the truth after the fashion of western historians labelling other cultures as savages and barbarians, but this doesn't detract from the fact that it is well illustrated and well told.
It is necessary to remind yourself that this is a work of fantasy, though i believe Frank Miller does the Spartans a disservice in his portrayal of them, and especially of Leonidas (in reality it was because, not in spite of the oracle that he went to war). The portrayal of the Persians is fantastical, but fits in with Herodotus' legendary accounts of dog-headed men and monsters and is justified in that the story is told from the point of view of the Greeks.