We quite enjoyed this one as a popcorn movie. In fact we watched it on our wedding anniversary, so let's call it a great date movie.
I think you have to watch it as a Western, not an SF movie, in order to appreciate it. There's really not much to it. There are some cowboys (and indians) and some aliens, and they duke it out. End of story. It's pretty to look at, fun and forgettable.