It's...just not very good.
I like the premise, I really do. Cowboys and aliens and dinosaurs? How can you not like that? And I do appreciate the author's attempts to weave in real historical figures and events. But overall it is very poorly executed. Characters are fairly hollow. The pacing is poor, switching between long, arduous exposition and action sequences that happen so fast and with so little description that its over just about as soon as its begun. The main "antagonists" are the alien characters, but they are very poorly constructed. Their motives and behavior seem to morph to whatever the scene calls for at the time, and their relationship with each other is perplexing. The background on the aliens is a little more explained in the second book, but if they were introduced in this book then there should have been a little more info here. As it stands, they just seem to run around, advance the plot occasionally, and then disappear at the end.
I can forgive the poor writing execution, since it is a relatively early novel for this author, but I cannot excuse the extreme lack of any sort of editing or writing guidance that should have helped tighten a lot of these issues up. For example, there is a scene toward the end, where a character is very explicitly described as disappearing into thin air, its a major point in the entire scene, and then IN THE VERY NEXT PARAGRAPH that SAME character is described as being part of the background. I kid you not: page 298, first two paragraphs.
All in all, some good ideas, a couple well-worded jokes, but really not worth the time overall.