Yes, I've liked Sin City. However, I've only watched it from the second attempt.
When I turned it on for the first time and watched the first several minutes, I've just closed the window and deleted it from my hard drive.
I didn't like the style - black and white with speckles of red, it looked overall idiotic to me. Also, the name of Tarantino (as an invited co-director) in the beginning of the movie put a giant cross at the whole movie for me. I hate his movies and style, they're pure trash. I've marked the movie as crap and forgot about it.
After about a year, I've started noticing people talking about it. Not that cattle, who liked the Transformers, but normal people with their own opinion. I've thought "well, even if they liked it, maybe there is something there" and downloaded it again. Then I've finally watched it and I was impressed, the movie has turned out great.
So, yes, I like it. And I'm waiting for Sin City2, which is scheduled for 2012.
Unbelievable? Well, if you refer to the environment that mingles fact and legend, even King Arthur is unbelievable.
You didn't get it, totally. It's even strange for me that you didn't recognize this linguistic construction due to your occupation.
X, that's unbeleivable.
It's so boring, that's unbeleivable. = It's unbeleivably boring = It's hard to beleive how boring that is.
Also, that is commonly used in the South Park against Cartman:
You're so stupid, that's unbeleivable.
You're such an asshole, that's unbeleivable.
And so on.
What about the 300 movie. I've liked the visual style, the voice of storyteller, Leonide (this is SPARTAAAAA!!!!), but I've found the movie itself so boring, that I wished it ends as soon as possible. At least, I won't watch it for the second time.