Winter of Atom has a good story, optional survival mechanics, the beginnings of a settlement/reputation system, and a few pages that make combat encounters clearer, but I bought this book primarily for the new Gen 3 Synth player character background, given their importance in the world of Fallout 4. There are a total of 3 new backgrounds in Winter of Atom -- Gen 3 Synth, Protectron, and Child of Atom.
When the new Settler's Guide Book was released a few days ago, I quickly purchased it for the same reason -- the new character backgrounds, weapons, and craftable items to add to the core game -- only to find that it, too, contained the Gen 3 Synth and Protectron backgrounds, settlements/reputation covered in greater depth, and even the Salvaged Assaultron Head weapon stats (one of the big selling points of the Rust Devils content).
If, like me, you are mainly interested in system additions (characters, weapons, perks, etc) then my recommendation would be to skip this title and get the new Settler's Guide Book instead. I wish Modiphius provided more detail on their roadmap so we could get an idea of overlap within the published and upcoming Fallout 2d20 books -- it's frustrating to pay for the same content twice. Changing the statement "Create characters with new origin options: Generation 3 Synths, Protectrons, and Children of Atom" in the product description to "Winter of Atom contains previews of the Gen 3 Synth and Protectron backgrounds from the upcoming The Settler's Guide Book" would have let me know that I could wait for the next book.