EDIT: After seeing Vol.2, my rating has changed from 4 to 5 stars. Please read the review of that Volume for more details.
Spurred by my enjoyment of Inglourious Basterds, I decided to give this movie another shot. Don't let the 4 stars mislead you, heading into this with the right mindset made a gigantic difference; I absolutely loved this movie. It's an exercise in badassery and style, and is gripping from the first frame to the last. The music is genius, the action is thrilling, the acting is quite good and the script is brilliantly written.
Here's why this doesn't get a 5 star rating, then: despite all this, the movie just seems like one huge fight scene. Not that that's bad per se, as it's all done in a masterful way, but that still denotes a certain lack of substance (the story could be summarized in two or three sentences). As fun as this movie is in its unique styles and references, there's nothing under it all.
Basically, it's a genius summer action flick, not a genius film.
In a sentence:
Nearly unmatched in uniqueness and style, but somewhat lacking in substance. Nonetheless, this is a must-watch.