Sin City is by far the best movie I've seen in recent memory. I'll be hard pressed to find anything negative to say about it.
This is the first movie based on Frank Miller's Sin City graphic novels. One of the goals for the movie was to bring Miller's unique style of storytelling to the big screen. There has been a big run of comic book-inspired movies recently, yet most rely on characters and/or special effects wrapped in a traditional Hollywood movie package. Few films have attempted to capture the unique world of comics (or graphic novels if you prefer) and their distinct look or abbreviated dialog. I can really think of one other decent attempt before -- Warren Beatty's *** Tracy back in 1990. Although I wasn't bowled over by the acting, it was pretty successful in bringing a Sunday-style comic strip to film. Sin City raised that bar significantly in every way. To achieve the look they were after, nearly everything in the film other than the actors, was computer-generated, yet you don't notice at all. I think Sin City will be the standard for all comic-genre films to come.
Special kudos to Frank Miller and Robert Rodriguez for their brilliant direction; Mickey Roarke, barely recognizable as Marv; and especially Carla Gugino for making the very most of her minor role as Lucille (she's been a personal favorite of mine for quite a while now).
Rotten Tomatoes: 78%
P.S. - News of plans for both Sin City 2 and Sin City 3 recently got out. I pray for more Carla Gugino!