For one thing, the setting is great. There's a lot of room to move and be creative, but because the Old West setting is so familiar, it's still very easy to get into the story. Most readers probably won't need much introduction to Doc Halliday, the Earps, and the rhythm of life in Tombstone, while the galactic background that brings Macklin to Earth holds hints of great promise for future developments. The beginning of the book is a little choppy, with some rather blatant historical details dropped like anvils into the text, but the writing smooths out fairly quickly as you get caught up in what's going on. The tension as they close in is well handled and there are a couple of rather unexpected twists in the story arc.
Historical events complement the story rather than dominating it, something that's very nice to see.
The action motors right along, which does unfortunately mean that characters and dialogue are at times a little under-developed. Mostly this applies to the human villains, the Clanton gang, who tend to come off as cardboard cowboys without much to distinguish them from each other, but Macklin does have HERO stamped in big letters across his forehead. When the characters do fall down, though, it's not bad enough to break the flow of the book. This isn't an epic saga and I don't think it pretends to be. Put your mind in neutral and enjoy the adventure.