I can't explain it, but this book really stuck with me. Granted, it's not a life-changing book, but merely what it advertises: a work of hard science fiction. Although that genre set the expectations pretty low, I found this a readable and entertaining book - with a couple of "I coulda done that" heroes that draw you in and take you along with them.
The book really works as a great alternate history tale. At the start, it is comparable to Modesitt's "Tangible Ghosts" in that you can tell something is different from our world, but you're not sure what. By the end, these little mysteries are resolved in a greatly entertaining fashion. All in all, I put this book down feeling - very satisfied.