My dad and I went to see Cowboys and Aliens (genre mash-up!) the other day. It was not horrible. The genre is both hindered and made more interesting by the fact that people in the Wild West haven't even the dialogue to comprehend aliens. This effort, however, bit off more than it could chew and seriously over-plotted. While there may be potential, I think the western/sci-fi hybrid has a ways to go.
And, sadly, Eureka got cancelled :(