With a title like that you would probably expect something a little silly, a comedy or even a spoof. But this movie is not silly or light-hearted at all. Without the aliens it would be a decent character-driven cowboy movie. Without the cowboys it would be a decent sci-fi action/horror.
As it is though, it's somehow less than the sum of it's parts. As a cowboy movie, the importance of the human issues and relationships seems unimportant next to the threat of the aliens, but the "western" movie structure doesn't work all that well for the alien angle. At least that's what I've come up with for why it doesn't quite work. But there's some more concrete issues as well.
The setting, while novel, puts the humans at too much of a disadvantage - not only do the aliens have superior technology and weapons, they are also larger, much stronger, faster and more agile than us, easily capable of climbing walls and roofs, taking down a horse and rider, killing a man with a single blow, and shrugging off mass quantities of arrows and bullets. Against such odds, the eventually human victory is simply not believable, and the movie falls apart.
Another issue is that the aliens themselves, as the 'bad guys', don't have any real character. That's not to say that they aren't well designed; I thought they were quite original. But consider Predator; while the Predator was barely seen for most of the movie, it all came down to confrontation between it and Arnie at the end - same for Alien and Sigourney. And while Star Wars is arguably a better comparison in that it has large battles and a more extensive cast, it still comes down to Vader, Luke, Obi-Wan and the Emperor. The aliens of Cowboys & Aliens don't have the personality and presence of Vader, don't dominate the movie the way the Xenomorph and Predator do. In the end, they are distant and faceless.
There is one single recognisable alien in the movie to serve as Daniel Craig's nemesis, but the fact that it has a scar fails to elavate it beyond the pack, and it's eventual demise doesn't really have much weight. All in all, while the aliens do present a very credible threat, they are unfortunatley distant and rarely seen - even in the large 'final battle' there's usually not more than one alien on screen at a time - which robs the final victory of it's gravitas, it's importance.
Having said all that, the characters themselves are generally believable, some are even likeable, and their personal stories do matter to the viewer. Unfortunately the protagonist himself is trying so hard to be a strong, silent, square-jawed hero that it's hard to actually form a connection with him, even though he get most of the screen time (hell, even the character's name is Jake Lonergan). Nevertheless, Daniel Craig handles the action scenes and the meagre dialogue well. Harrison Ford perhaps has the more interesting character; if you're expecting Indiana Jones, you'll be disapointed, but the character has depth and Ford does nail the role.
The special effects don't feel out of place, the aliens are well designed, and the action is entertaining. The story is pretty good overall - at least considering the fact that the title came first and the plot second. In fact I would say it has one of the more believable reasons for an alien invasion, although there's a few too many things left unexplained for my liking.
Overall though I'm going to give it a 6 out of 10: it's not bad, but I can't find it in me to say it's good.
#####SPOILER WARNING#####I mentioned the movie has one of the more believable excuses for an alien invasion. The way I see it, mining for a precious mineral is quite believable. I don't know how prevalent gold is in the universe, but I'm guessing it can be found in non-inhabited planets, so why come to an inhabited planet? Economics. If the economics of mining the gold makes it worth it to come into conflict with the natives, then that's what they'll do (we do the same on a country basis all the time). Clearly earth matches their home planet environment, else they would need space suits or something, which could be a significant factor. Gravity, temperature, so many factors could interfere with the refinement and related processes, that not just any gold-containing planet would do. Humans are probably the least of their worries.
What the hell was up with that boat? Oh, and why are the aliens naked? We don't send astronoughts into space naked. How come they can take so much abuse? Those are bullets, not spitballs.
But most of all, if the humans have one single weapon capable of actually harming the aliens, and at least several, if not all, of the aliens have similar or better weapons:
a) how do the humans not get instantly annihilated?
b) how does Jaker Lonergan manage to so easily take out several dozen of them, when they have the same weapons and should be more familiar with them?
c) why the hell don't more humans pick up alien weapons when they drop them?