In much the same way that "Let the right one in" has been described as a film about children that happens to involve vampires, I would characterise Cowboys and Aliens as a film about aliens that happens to involve cowboys. There is very little about the story that could not have been transposed into modern times, or even switched to involve any cultural group by simply changing the language, weapons and location. What I guess I'm trying to say is that there seemed little reason for the people involved to be cowboys, beyond the snappy title and a couple of half-hearted parallels around the appropriation of resources from natives.
This could have been an interesting opportunity to examine the cultural differences between modern and "wild west"-ern societies through the medium of their respective reactions to alien encounters. Alternatively, the film could have attempted to flip the traditional colonisation view, by having the pioneer society under threat from an imposed alien culture. Instead, this potential is lost as time is spent on the exposition required to cover multiple story strands and include additional characters, none of which are particularly interesting. In addition, once the climax is finally reached, the film descends into pure CGI-powered action-film nonsense.
Daniel Craig does well as the central figure, retaining what little mystery the film hangs onto with his strong, silent visage, but the rest of the cast is pretty average. Even Harrison Ford is given little to do, and as a consequence of the number of secondary characters floating around, is never given a huge amount of direct screen-time with Craig and certainly not long enough to establish an interesting relationship. There are some minor plus points, mostly around the visuals, which manage to blend the alien entities into the action quite believably, without having to resort to shadows and half-shots.
Overall, while not being totally without merit, the film certainly never rises above the most obvious implications of it's high concept starting point.