Sin City is slick and violent and over-the-top in every conceivable way, from story to prosthetics to set design. It's dark and grim (although not entirely humorless); despite its portrayal of exceptionally strong women, it's misogynistic (the strong women are either scantily-clad prostitutes or scantily-clad innocents dressed like prostitutes). It's every bit as cartoony as the comic from whence it sprang.
And yet, that's exactly what makes it so good. I harp on about strong sense of place being my favorite feature in any particular feature, but really, that's what a great movie does: establish its own universe and hew to it like a motherfucker. Sin City is its own sleek, black-&-white (with carefully chosen flashes of color) world, darker than the darkest live-action noir, crafty and shifting and full of delicious evil and the chumps chosen to fight it.
It's not a nice world; it's not a place the squeamish will want to visit. Don't expect any big emotional involvement, either; this is a big, fat, gorgeous cartoon, and maintains a kind of formal distance as such. The Boyfriend called it disposable entertainment, and it is: for better or worse, it's not going to stick with you (and considering some of the grotesque imagery, that's probably a good thing).
But it is spectacularly realized and engaging and worth seeing, for the non-squeamish, on the big screen. With maybe a DVD rental upon release to check out what's under the hood...