I read this, as i did Return to Oz, because I like Joan D. Vinge's writing and there's not enough of it! And while I'm not sorry I read either book, they are not as good as her original works.

As a novelization, I think Cowboys & Aliens is the better of the two. It reads more like a novel than a novelization, and it gives a lot of background and context that I think would have been impossible in the movie. (Note: I have not seen the movie, nor do I plan to.)

If one is a fan of the movie, I think this would be an excellent read that would increase one's appreciation. However, as a novel in and of itself, it's a bit heavy-handed, especially in terms of plot. Plus, I still cannot see how it makes sense for aliens to come all the way here for basic resources, when they can -- with their advanced tech -- just MAKE whatever they need. Most Earth lifeforms have a innate policy of doing as little work as possible to get the results they want -- I can see why this would be selected for -- and I do not see why aliens would follow a wildly different, more complex and far more fragile evolutionary path. But hey -- it's mostly a plot in which Humans Triumph and Virtue Is Rewarded, not actual science fiction ... more a morality play with SF garnishes.

It's very well-done as a novelization, though.

Also check out the original graphic novel, on which the movie was based.