I may be biased as a I'm a huge fan of the Prophets and have been looking forward to this book for a long time, add to that the authors and... well, this book had a lot to live up to!
Did it live up to it and more! Everything you could want to know about the Prophets for playing one, how they act, their prohibitions, their philosophies and everything else. But far more than that it explains their history and the reasons they came to be in a way that really lets both GMs and players understand where they came from and where they aim to go in the future to some degree.
I won't go into spoilers but... damn, the information about the accuracy or not of the Prophets beliefs is fantastic, it's incredibly well thought out and meshes with everything we've learned so far really well.
If I had any complaints it would only be that there might have been the opportunity for more feats, etc related to the subject matter than we actually see, especially given the fact it's the last PF1 book we'll get on the subject. That said... it's really hard to even feel bad about that as the book is so well written I can't imagine what I would want cut to replace with rules, hell, I'd happily have bought it with an extra 20 pages of fluff and rules in it!
tl:dr; If you're interested in the Prophets or the Mercenary League this book is for you, all that might be expected and more (plus a wealth of GM secrets and things they can work into plots, more than enough to set an entire campaign here).
I tend to run a lot of games to introduce players who are new to RPGs to Pathfinder, a lot pick it up quickly but for some it can take longer, there is no better book for all of them than the Strategy Guide. I'm frankly amazed how well it's written, it's basically the Core Rulebook for new players in a far, faaaar more easily digestible format.
Every new player I've sat down with this book has come out of it with not only a solid character that actually fits the idea they had, but a much better grasp of the rules than those who don't, it's my go-to now for any new players.
The scenario has an excellent flavour, Nidal is suitably unpleasant and the Cairn is very well described and does a fantastic job of evoking a Hellraiser feeling.
Unfortunately that's about all that's easy to compliment, mechanically the rules for sight, movement, traps and just about everything else are going to cause issues. Not simply in that they seem rather harsh with respect to low tier players, but also they are contrary to how such things usually operate and require both the GM and players to quickly adapt to this, for no real gain that I could see.
There is also an issue with essentially removing treasure from the players after successfully dealing with a trap, which seems rather unfair. The scenario in general also has numerous points where it is very punishing for non physical characters, requiring numerous skill checks from them if they wish to participate and which failing these checks poses significant risk that seems out of proportion to what might be expected.
I genuinely enjoy the atmosphere of the scenario a *lot*, however I can't recommend or highly rate it since all the problems listed create a very difficult environment to enjoy the actual gameplay, there are also some map issues with it seeming like at least one room is not depicted and things that seem to be depicted not mentioned in the text.
Overall a great concept but let down by far too many mechanical problems and the potential to excessively punish many characters.