I'm a huge fan. 300 was long one of my favorite Frank Miller works, only behind TDKR. While everyone was clamoring for Sin City to be made into a movie, I was sitting in a corner clutching 300 and hoping.
I remember how skeptical I was that they could pull off the graphic novel translation to film convincingly; I just wasn't sure the audience would buy that these guys were meant to be paragons of warriors, stripped down of armor in the graphic novel because they honed their entire bodies into weapons. Frank Miller himself said as much, that it's nowhere close to historically accurate, but isn't intended to be. Not history, but based on history and more akin to a myth or legend handed down over generations. That's a huge facet, hence the role of Dilios in the story. As a huge fan of the GN for so long, I was delirious with joy as to how Snyder translated to film. It's not meant to be Oscar caliber, it's not meant to make you think, what it is meant to do is portray fighting as honorable and even almost balletic in the way it can be orchestrated by skilled fighters... and Frank Miller did get that right. Hell, Sparta was the only Greek city-state that lacked walls; they believed the men of Sparta would be defense enough; and what's important to note is that they were right, considering what's known of battles in which Spartans fought, their culture, and the Agoge. If you're at all interested in the period and want to read an EXCELLENT story about the Battle of Thermopylae, I recommend the following...
Easily one of the best "surprises" I just picked up on a whim and became totally engrossed in. If you're into that era, it's an excellent book.
ANYWAY... I guess what I'm saying is that Snyder has earned my confidence that he can pull off a faithful adaptation with a degree of creative liberty. That said, 300 wasn't meant to make you think, and Watchmen is intended to be extremely thought provoking. Therein lies the crucial difference.