Please answer me this: what do you expect of a movie with such a title? If your answer is, by any chance, an action flick with a serious plot and high-budget in no way related to corny animations, dialogues or B-rated special effects, please step aside from this description.
The main issue with this movie is that not only fails in delivering an engaging flick filled with fireworks and explosions, it actually tries to be up there among other seriously well-done movies: it tries to get a story, and it fails. It tries to put interesting dialogues and speeches with extremely good actors like Daniel Craig or Harrison Ford, and it fails because their roles does not favor their qualities (they could even put 'generic actor A' and 'generic old-man B' to do their jobs, no one would ever note the difference), and it even tries to make the mixture of cowboys and aliens something that could even be... plausible? Seriously, the "plausibility" of Olivia Wilde depicted as a clean and white-skinned lady in the dirty wild west is the personification of unplausibility and fantasy.
If they'd just try to do a B-movie out of this, with unknown and hilarious actors, with a purposedly childish plot and jumping-the-shark moments, perhaps it would get a cult following (with lower-priced ticket prices if possible, enabled by lower budgets for such a flick, but I don't know enough movie econimics to comment on this). As it turned out to be, Cowboys & Aliens is a waste of money and misused talent.