Welcome aboard! Always happy to have someone new sign up. What is Darwin's World like? I glanced online... Post-Apocalyptic and it's run off the Open d20 or does it have it's own system and DW is just a world setting with maybe a few rules tweaks?
Hope you have fun! The board has been a bit quiet lately but hopefully that will pick up soon. I'd love to run a game but my not so new anymore job has me running around too much to even consider it. I wonder if the unemployment line has free wifi... hmmm..
If you like the PA genre, which I do, Darwin's World is a great place to be (...err, visit)! The default setting is The Twisted Earth, and it's pretty grim. The setting is set primarily in the southwest United States, many, many years after the Final War. Mutations abound. There are several different factions struggling for control of strategic and/or barely hospitable pieces of land. Obviously there are ruins everywhere. And radiation; let's not forget that! There is some high-tech, futuristic weaponry in the setting but it isn't
prevalent, like Rifts. It does serve to illustrate how far the Ancients had advanced before their self-destruction though.
There are a few things to consider though if you intend to pick up some of the material. For starters, The Survivor's Guide, The Campaign Guide, and Terrors of The Twisted Earth (bestiary) are sold separately in pdf format, but they're originally different chapters of the same hardcopy.
The Survivor's Guide (chapters 1-3) is not setting specific, and deals with character creation. It's like a post-apocalypse character toolbox. The game uses the d20 Modern rules, but the designers have made the appropriate rules modifications to reflect the genre. For example, some skills or backgrounds don't exist, but they have included new ones or modified the old ones to work in the setting. The equipment section includes contemporary weapons and sci-fi weapons, armor, and vehicles. IMHO, you can pick up the core rulebook and run Gamma World, Mad Max, or even Fallout.
The Campaign Guide (chapters 4-6) serves up The Twisted Earth setting, but it also has a ton of information to run other kinds of campaigns, based on your chosen demise (nuke, plague, social decline, etc.). This book is where you'll find Environmental Hazards, Adventure Locations, etc.
Finally there's Terrors of The Twisted Earth which is your bestiary.
There is a lot of supplemental material for the setting as well through the Post-Apocalyptic Dispatch. They've produced even more character options like Sleepers, Vault Dwellers, Beastmen (think d20 Modern Moreaus), more vehicle rules, more locations, etc.
I wholeheartedly recommend it, but others may feel differently.