Just got mine yesterday.
So far...
I'm a little iffy on the plot premise (demon lords are going to unite in a demon council... yeah right.) That said, I notice that if you work with a cosmology that doesn't make the abyss into an "epitome of chaos", it might work.
I'm also thinking (spoilers):
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At 10th+ level, it seems like having a patron who is employing the PCs for false pretenses is a plot hook that can easily go afoul if the players get suspicious... and history tells me, they will.
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I'm not a fan of the new stat-block format, but I guess that's inevitable at this point. The encounter format, I'm actually okay with. I think it will bloat out the book a little, but it does make it easier to pilfer or cherry pick the encounters, which is sort of my MO with official adventures.
Still think the cambion write-up is crap. I think I'll substitute the ToH cambion.
Looks a bit combat-heavier than I was hoping for for what is essentially a PS adventure. There is an "abridged version" outlined, but it seems like the abridged version is more combat heavy and sidesteps some of the sort of problem solving/investigation type play that I find essential to an enjoyable D&D experience, and would make the adventure, if shorter, more of a slog.
Presentation is really nice, and from my first browse-through, there are some interesting and creative encounters.
Much more to read, but that's my first impression.