Darwin's World 2 is Gamma World without a lot of the "wahoo" elements like really crazy mutations. Like GW, just about anyone you meet is a mutant. There are sentient mutant animals just like in GW as well. However, unlike GW, DW2 takes a more gritty stance rather than the "cartoony" feel of most of the GW editions. There are super-science items in the game, but most folk are using spears or weapons crudely fashioned out of scrap. One of the best pics from the early version of DW was a character using a stop sign as a shield. It's more Mad Max than GW's Thundarr feel. Overall, though, DW is a smarter system that can be used as a good base to build up the more wahoo elements (which I love!).
Plus, it's a HUGE book crammed with a lot of crunch and fluff. Basically everything that you'd need to run a PA campaign all on it's own. I'm not content with just that though, so I add in a lot of info from other d20 Modern games like Urban Arcana, Blood and Brains, Blood and Fists, GW D20, and D20 Future.
Kane