This book was announced in June and I just discovered it today- what rock have I been under. Anyway...Excited is an understatement. I'm a sucker for all things fey, and I've been chomping at the bit for a book about the First World.
I do have a question if anyone can help me with it- I've read some mentioning that this book would be useful with the Kingmaker adventure path. I was planning on running Kingmaker before diving into Strange Aeons, so survivors of that AP and other past APs can have a cool new kingdom to retire in since from what I understand about Kingmaker from the description, it's the AP to create a new kingdom. So, should I wait for First World before starting King Maker? All four of the players' characters are fey(ish) themed, or could be enhanced depending on what's in the book- fey and harrow bloodlines, fey trickster mesmerist archetype from UI, and a witch with a First World patron.
Depends on what you're looking for out of it. According to what I've read earlier on this thread, it seems like there won't be too much mechanically, as that apparently is the case for most Campaign Setting books, although Feysworn still looks fantastic. If you're looking for new character options, maybe just let them retrain when the book comes out.
However, given the lesser amount of mechanical goods that seem to come with this, it seems like there will still be a huge amount of lovely fluff which you and your players can make use of, you to build/enhance the world it takes place in, and for them to use to build their kingdom.
tl;dr: If you're looking for mechanical stuff for your campaign, just start now. If you want fluff, you may want to wait.
At least that's what I've gathered from comments.