Saw this a few days back. It is, in my opinion, a great movie in the sense that it is not only exactly what you expect, but satisfyingly so. Slight spoiler alert the whole thing.
Cowboys vs. Aliens is not a dramatic tragedy. It is not a romantic comedy. It is exactly what it sounds like. Take some cowboys and throw some aliens in. It watches like a fourth-wall-shattering masterpiece in which aliens interrupt some epic western movie, then everyone teams up to fight it. It does this greatly so- the battle scenes are well done, and the special effects are absolutely phenomenal. The story is kind of generic but entertaining and well-suited as its purpose in a movie of this sort is to transport the viewer between action scenes.
There's a great series of subtle statements in the movie as well, such as that later on, spears and Indian weapons which the cowboys often regarded as worthless are the most effective against the aliens. Then, there's a series of cinematographic choices which make an interesting parallel between the weapons and methods of the Indians being easily adaptable to and similar to those used by the medieval fighters of the past, in the example of a spear being used as a lance effectively.
There's also a callback to the old westerns in the use of Harrison Ford as Woodrow Dolarhyde. It's reminiscent, seeing him in a western, although kind of odd that he wasn't starring in it. It was for the best I suppose.
There's a lot of fridge logic questions that arise from this movie though. For example, why didn't Alice ever change her cloaking? It would make sense, for example, to change into one of the aliens to disguise herself when she was in the core at the end, as she would have been both more agile and less noticeable as a target. Also, when pistols proved to be almost completely useless against the aliens, while rifles did have an effect, why didn't any of the cowboys switch weapons to better effect?
Overall, I'd recommend this movie. Watch it if you haven't.