I'm not too crazy about alternate history, but Progenitor is easily one of the most creative takes on the superhero genre. When I say "creative take", I refer to every aspect of "super" posited in Progenitor - manifestation and transmission of powers, the impact of said powers on history, etc. I wholeheartedly recommend Progenitor for the underlying concept alone.
As far as "crunch", new game mechanics outlined in Progenitor, it mostly encompasses metahumans throughout the Progenitor timeline. There are also rules for Syntergenics, which are basically super-contagious memes. But what really impressed me was the STEW gauge - Science, Technology, Economy, and War. It provides a metric to measure the impact of the PC's actions on a global scale. When any aspect (or combinations thereof) become dominant, there is a shift in global disposition. As with the Progenitor storyline, I absolutely loved the STEW system and heartily recommend Progenitor for this innovative scale.
So, why does this book not have a perfect rating, given my glowing praise? Well, it simply feels that everything is TOO well established in Progenitor. I am aware that we are free to select and incorporate whatever we wish from an RPG text - these aren't video games, after all. But after reading Progenitor, I found it difficult to devise any potential campaign arcs beyond the examples provided. Progenitor would make for an amazing television series or movie, but it seems to me that one is better served poaching concepts and mechanics from this book and creating a setting of one's own.
Progenitor does make for some great reading, though. Regardless of what you plan to do with the book, it is fun to simply skip to random page and read a metahuman profile or blurb of alternate history or whatever.