My gaming world of choice, like many people, is the one I started with. In my case, that was Greyhawk. I recall staring for hours at the giant fold out map from the Greyhawk Gazetteer that was taped to my bedroom wall. Every hex held the potential for adventure. For many people, especially the younger crowd, their world was Faerûn, AKA the Forgotten Realms. By looking at the sales of our D&D 4E Dark Sun books, many of which were sold to people I know don't even play 4E, Athas holds some nostalgia. Campaign settings are really important to gamers, so the Paizo world of Golarion had some awful big shoes to fill. Some of the Paizo designers worked on Greyhawk, so my guess is they have similarly strong feelings towards setting design. In any case, how I feel about Golarion and my personal setting baggage will color a lot of what I say about it, and your baggage will likely flavor how you'll receive it.
What strikes me most about the Golarion setting is that it's designed to be functional. Here's my confession: I honestly don't care much for the Golarion "good guys," the baseline nations and cultures that underlie the place. However, and this is an exceedingly big however, man is Golarion chock full of adventuring potential. While I enjoyed the many semi-realistic nations of Greyhawk, adventuring in that setting was on the margins, in the interstices of cultures and nations.Golarion, in contrast, has great swaths of African-like jungle, Egyptian-like desert ruins, frosty barbarian reaches, frightening ocean going trade routes, seas of quicksand and Atlantis-like lost worlds. Adventure staring you right there in your face.
From a GM perspective, Golarion is incredibly useful, even if as a whole it doesn't quite come together like what I'm used to. However, as a mature gamer, I'm free and capable of using what I like and discarding the rest (cough, Numeria, cough). I wish I could bottle and sell that sentiment. I mention this because many people I know discount Golarion because it doesn't have the feel they're looking for. I'm suggesting they take another look and consider the parts, even if the whole is less appealing. There are some brilliant bits.
So what about the actual Inner Sea World Guide? I've been reading the various Pathfinder Chronicles books, such as those on religions, factions, races and even the smaller gazetteer, so I was hesitant to pick this up for fear there would be a lot of duplication. This can happen with some of the books, especially when you endeavor to become a SME (Subject Matter Expert) on one Golarion topic or another. In the case of the Inner Sea World Guide, there is some duplication, but I was pleasantly surprised to find a lot of new stuff I hadn't read before in just about every category.
Sometimes new things are small, like how the section on races provides recommendations on non-standard PC races and ones to consider carefully, or how how the deities are assigned regional origins, making them feel much more organic than before. Sometimes additions are huge, like the inclusion of First World (faerie realm) entities or Cthulhuesque gods, or the speculative map of the rest of the world on page 204. That map alone was what put the book on my radar. How about a cosmology that includes inhabited planets? There is some "wow" stuff here for sure, even if the descriptions are sometimes place holders for future books rather than complete entries.
Like the enormous and exhaustive Inner Sea Regional Poster Map that Paizo recently published, the world guide feels equally complete. You get the sense the team scoured the over 150 Paizo products to include all the content on the section in question. There are languages you've probably never heard of, an exhaustive section on the gods, and of course the gazetteer section itself, which covers 182 of the 300+ pages. This is the section that really shines, and probably why you're likely to buy it. It's incredibly dense and encyclopedic, while still being delightful to read straight through.
If you aren't a completist and don't plan to buy every Pathfinder setting book, this tome will serve you well as the ultimate gazetteer, and a highly functional guide to the cultures of Golarion. In this sense, The Inner Sea World Guide is an ideal "core book" for running a Golarion campaign, including the adventure paths. It puts a lot of the rules and options into cultural perspective, which can get lost, especially amongst your power gamers. Besides being the definitive resource on geography and culture, you also get about 50 pages of rules content, including updated prestige classes, new spells, equipment, magic items and monsters. Like most Paizo products, this is highly functional content, put there for a reason, as opposed to filler. This is the typical Paizo value add.
The Campaign Setting that The Inner Sea World Guide replaces was going for over $200 on Ebay before this release. It was a 3.5 book with content that needed to be updated for the Pathfinder RPG. The new book includes an addition 64 pages plus a bunch of new content added when older content was replaced or eliminated, often because it was covered elsewhere when the Pathfinder RPG was first published. The book is $49.99, but it's a treasure trove of content you're likely to reference for years to come, or read through completely for the fun of it.