Cowboys & Aliens is pretty good fun as long as you don’t go into it expecting a comedy. It’s mostly a homage to classic Westerns and their tropes mixed in with an 80s-style practical alien movie and its tropes, especially playing with the ones that overlap. It acknowledges the silliness of the notion and proceeds from there.
(The aliens themselves are nicely done, sort of apelike and sinewy like the Future Predators from Primeval with faces oddly reminiscent of Mutts from Doctor Who. They have a decent sense of threat even in the final battle.)
As cliffc999 on RPGnet noted at the time: “What I found most interesting about the movie is that in any standard cowboy movie, Harrison Ford’s character would be the villain.” As director Jon Favreau notes in the commentary, people putting aside their differences for a great cause is part of Westerns and alien invasion movies.
Meanwhile, I still want a historical Predator movie.