Okay. I need to say that this is one of the best movies I've seen this year. It's really solid. Rotten Tomatoes has it in the 90%+ range right now, and it deserves it (especially compared to craptastic movies like "In Time"). About the only complaint I have is about Joseph Gordon-Levitt's eyebrows -- they were far too dark. If I could change anything about the movie, it would the black percentage in his brows. Which of course only highlights how decent the movie was, if that's all I would change.
It's essentially a gritty action movie with some decent laughs scattered throughout, along with some scifi elements. It's a testimony to the director that there are some pretty harrowing scenes that are interwoven into the same movie that has some decent chuckles, and it all still works -- he nails tone very well. He also doesn't pull punches or hide the results of violence; you will see some rather unsettling things occur (one of the worst involving Joe's friend Seth).... One can riff off the time travel meme and come up with some pretty nasty results -- but even those kinds of things contribute to the overall plot, so you can't say a particular twist was unexpected.
The cast is pretty solid, but I have to take note of the boy (Pierce Gagnon), who is dazzling, especially considering how young he is. And you don't want to see his "ugly" face.
EDIT: Oh yeah, this is one of the few movies I've ever seen where the audience was rendered speechless in the last two minutes. No one had any idea how to respond to the ending. It's one of those things where you just don't even know what to say, you just find yourself sitting and staring at the screen as the credits roll and just not coming up with anything.