Haha, most reviews criticize the script. Amazing. Apparently, what was good enough for Star Trek or Iron Man (can ignore plot holes it as long as it is exciting) isn't good enough here.
Well often if a movie is fun or stylish or charming enough (as those two were), it's a lot easier to overlook the story problems.
If a movie like Cowboys and Aliens is completely lacking in those areas, you're only really left with the story
to entertain you. And it better be damn good.