"Cowboys v.s. Aliens" is a modern telling of a traditional Old West story: Man wakes up in desert with no memory. Man gets to town and causes trouble. Man saves townspeople from aliens, regains memories, and helps townspeople fight the aliens.
What makes this movie so extraordinary is that the director manages to make an action movie with subtle character interactions. Daniel Craig, who places the man with no memories, does a fantastic job with his character's personality. The more he remembers, the stronger the personality. Harrison Ford also plays a three-dimensional character. He's not a nice man, nor is he the typical Western bad guy. The scenes where these two men play off of each other sizzle on screen from sheer chemistry.
There is only one plot twist in the movie, with the rest of the plot guessable as you watch the screen. But the story unfolds so easily that you look forward to each scene, suspending disbelief without having to work at it.
All in all, I'd give this movie 4 out of 5 stars.