As others have said, this one is divided into two major parts.
The first part is a serviceable hexcrawl that takes place in the Nothern part of the Parched Dunes area the PCs started exploring the previous book. The goal, essentially, is to find the dungeon that is part two. This part is not particularly good or bad. The encounters are a bit ho-hum for the most part. I rewrote or discarded most of them to suit my group. There are also some optional encounters in the book that can be dropped in here and are more fleshed out and interesting.
The adventure falls flat on its face when you get to part two and the dungeon. The dungeon itself is not bad and has some fun,if occasionally excessively deadly, encounters. Where it falls down is the scale. It has a huge map done entirely on a 1 square=10' scale. Since the Pathfinder rules are written for 1 square=5', the map is actually 4 times as big as it first appears. The largest size Crystal Caste mat was not large enough for a small subsection of the map. I had to use a second, smaller mat to draw all of the rooms. This was less than a fifth of the dungeon. This map is just a huge pain to deal with. You either have to draw one part and lock the players in there or constantly draw, erase and redraw as the rooms they enter exceed the space available on your mat. The combination of a large map and the 1 square=10' scale makes things cumbersome and slow. The map is such a headache that I seriously considered just skipping the last part of the book entirely, cutting to the last fight, giving out the treasure from the dungeon, jumping the characters to level 13 and going straight to book 5 rather than hassling with it.
First, I would like to mention the quality issue. I have now finished running the first two parts of Wrath of The Righteous and both books have fallen apart. This is the worst of the two with large sections now falling out. The quality of the binding on these books has been disappointing.
What about the adventure itself? There are a lot of things about it that I like. It has a heroic feel that fits well with the theme and tone of the adventure path. I like the idea of the characters leading the counterattack against the forces of the Worldwound at the start of the 5th Crusade. It does a good job of using the NPCs, both heroic and villainous. There are a lot of good encounters and interesting twists. It sets up Drezen for later books.
There are a couple of things I don't like about it. The mass combat system is a novelty and it is overused early on. Mass combat just isn't that involving for most of the players. Several of the maps in it are very large and expensive to print to scale but largely unused. The Citadel Drezen map was huge and expensive to print but consisted mostly of empty rooms. The Gibbering Swarm map used earlier on is a prime example of a map that is far bigger and more expensive to print that is warranted by the few encounters on it.
This is the most tedious, most unfun and generally awful scenario I have played for PFS to date. Most of the scenario is taken up by a tedious escort mission with a suicidal NPC that seems to be built to make this as tedious and irritating as possible. I spent most of the evening grunting in exasperation and wishing the GM would just skip over this tedious crap to get to the end so I could not have to do this any more. All I wanted was for the tedium to be over with. I don't know why this gets such high ratings. I sometimes suspect that people who have been stuck playing it want to share the misery. I suspect it is rated so highly out of schadenfreude.