Great setting, reminds me of Star Wars meets Treasure Planet or something. My kids & I liked the (free) PDF so much that I actually went & purchased the physical book. I love the art & the concepts introduced here. My only criticism (hence not 5 stars) is a lack of detail. It would be great to have more example Aspects or Stunts for players to use, or more detail on the common technology available to characters or information on the various other cultures (like the Necrocracy mentioned in the adventure, sounds so cool!).
This is a common criticism from me towards most Fate supplements, so it is not unique to the Aether Sea. It's like saying, if you're familiar with the setting, you're fine & can come up with relevant Aspects & Stunts, otherwise, you need to make something up. Which is fine for some people, but for others, it means they're not going to be playing.
Still, a fine supplement!
[4 of 5 Stars!]