This book is fantastic, it's not for everyone, but it's genuinely amazing.
There isn't a single NPC that isn't in some way connected to another NPC and I shudder to imagine what would happen if you killed one of them.
When you start a fresh campaign on the Hot Springs Island, everything is in a perfect status quo. The moment you apply your players, everything will begin to topple.
Helping a faction can well enough lead to the extinction of another faction and you might think that "other modules have that too" but I can assure you that Hot Springs Island is very unique in its spiderweb of politics.
All factions and important NPC's have a "Want" and "Do not want" list, which makes it easier for you as a GM to keep track of NPC reactions to player actions, it makes the NPC's very fleshed out.
Watch as your players quickly realize that their actions have consequences, observe as their smiles contort into awe as the Island itself reacts to which faction they are helping.
Will they ally with one of the factions or will they work independently? Will they usurp the people in charge and become kings and queens on a paradise island? Will they fight heroically against terrifying monsters and trade their various bodyparts for services and goods? Or will they die overdosing on some strange plants while discussing the "nature of honor and sacrifice" with a lava hydra?
Your players can do this and it will make sense in context.
The book art is consistent, immersive and amazing. The book layout is phenomenal and with hyperlinks, it's very easy to navigate.
The rest of the industry should take notes.
Although, I would recommend against giving the players the Field Hand Guide, as I prefer to have my players explore the things in real-time. (I'll keep the Field Guide to myself, to read when I need inspiration).
The entire book is filled with tables and dungeon maps that together make for a very replayable module, I wholeheartedly believe that you could play the same dungeon 5 times in a row and it wouldn't be the same.
The creatures in this book aren't statted, which means that they aren't tied to any levels, abilities or numbers. This offers a great opportunity for you as a GM to take any monster on the island and make it a monster for your players to fight whenever you want. If you want a giant worm-monster to be a fifth level encounter, you can do that and no one would be the wiser.
This also means that the book is compatible with any system, but it's also compatible with any other setting since it's a self-contained island.
Do you want a Planescape campaign? You can start from the Hot Springs Island. You're out of ideas for your Mystara campaign? Turns out the Hot Springs Island was always on the map all along! Are your players done with the Hot Springs Island? Pretend that the island was always in Eberron and have them continue their adventure somewhere else.
If you don't need the Hot Springs Island anymore, the tables are sure to come in handy in other aspects of your GM'ing.
The Hot Springs Island is very much worth the price, very well done Swordfish Islands!