Princes of the Apocalypse is a 250-ish page hardbound adventure for 5th edition Dungeons and Dragons characters level 1-15, with an MSRP of $49.95 USD. Here's roughly what you're paying for:
13 pages of background, factions, and adventure hooks
21 pages on Red Larch and the surrounding Dessarin Valley, the area where the adventure occurs.
140 pages of keyed adventure locations, dungeons, and side treks.
24 pages of monsters and NPCs
8 pages on the Princes of Elemental Evil
4 pages of new magic items
3 pages on Genasi as a player race
13 pages of new spells
8 pages on adapting the material to various published D&D worlds.
5 pages of sketchbook and concept art for things that didn't make it into the adventure but really, really should have. I am definitely bringing the Walrus Knight back.
Note: these counts are eyeballed. The font size is generous and there is a vast amount of art in this book.
The book itself is Realms-centric, but only in the naming of certain factions and cities. It's slightly worse than what we saw in The Lost Mines of Phandelver. I'm not spoiling much by saying this adventure is about preventing an elemental apocalypse.
WHAT I LIKED:
The structure is pretty good. The adventure starts small, with a missing persons investigation, which leads to the discovery of one or more cults active in the area. The events will progress with or without the intervention of the PCs, and how they deal with them is a matter of choice. Combat is one solution, though infiltration and intrigue are likely to be more fun. For being nonlinear, the writers have managed to make the progression of events/monsters/cool locations feel earned.
Plenty of dungeons. Much like Zelda, there's a Water Temple, an Earth Temple, a Fire Temple, and Wind Temple. The Elemental Nodes are cinematic and evocative. The cults are just developed enough for the DM to make their own. There is a crapload of adventure content. Several of the side treks seem really fun. Actually, if you're starting a new campaign you'll need to run a couple off the bat, as the real joint doesn't really kick off until you're 3rd level.
It's not an adventure path. I wouldn't describe it as an adventure path in the Paizo sense because there doesn't appear to be any railroading (note: I understand that a certain amount of railroading is to be expected in an adventure path, which is why I'm happy Princes of the Apocalypse isn't one). It's really a campaign more than an adventure module.
Diversity of enemies. I feel like half the monsters in the Monster Manual show up in this thing. Some of them seem a bit randomly placed but it wouldn't be D&D otherwise. Also, over 15 levels you're likely to get mighty sick of 100% elemental-themed enemies.
The Balloon Pack: one of the magic items is essentially an air elemental-powered jet pack.
Shoutouts to The Temple of Elemental Evil are there without really being there at all. I always wanted to like that adventure but never really did, and I actually think I'd have more fun with this one.
WHAT I THINK IS REALLY DUMB:
The overland map. Once again, WOTC publishes a very pretty map gridded out in numberless 10-mile hexes. Which is good for determining travel times and that's it. There is ample dead space in this book to include 10 pages of encounters keyed to hexes. Even if you don't want to write them, at least number the fucking hexes for me, Mister Fifty Dollar Book.
What they cut. Those concept art pages have some pretty great stuff that I wish was in there. Giant vulture riders? Brain parasites? Arial dungeons? Who says no to stuff like that? (Answer: Chris Perkins)
A GENERAL OBSERVATION:
Like any book of this type, many DMs will want to tinker around to make the enemies and cults fit the world they're running. I think they've done a good job giving you an adventure that works without giving you too much that you won't use. Room and location descriptions are for the most part brief and manageable. These aren't One Page Dungeons, but I doubt anyone really expected them to be. This is always a hard balance to strike because everyone in the OSR knows you could do this book in a third of the page count, but then someone else will complain they paid for something that ain't finished cookin' yet.
BUT WOULD YOU RUN IT THO?
Yes, totally. I am not overcome with a burning desire to do so first thing tomorrow, but that might actually be a good thing. It's a solid, nonlinear, site-based adventure with plenty to do, and it stands head and shoulders above Tyranny of Dragons. I think this would be a lot of fun for any group looking for a lengthy, classic-feeling cult-bashing adventure. Many bonus points if they're also Avatar and Legend of Korra fans.
If you can't get down with insane cults who worship the concept of air or whatever, then this is probably not for you.
For everyone else, I give it a