The reason this story works is because the authors made it different in these ways:
(1) They put in actual methods to negotiate. So, rather than just bash everything, PCs can talk their way out or even make alliances. Additionally, not all the creatures of the Underdark are out to kill you so it allows more chances to talk things out. ;
(2) There's a great deal of context and History; especially in book one in the area of Haranshire (a backwaters shire); good to great character descriptions.;
(3) superb overland map you can share with your players (just don't show the back); include a few battle maps for key sections of the long adventure);
(4) Lots of overland adventures; some not just fighting but puzzle related or negotiation related; additionally, you get to deal with the locals;
(5) Great details of natural hazards in the Underdark;
(6) A variety of different monsters in the Underdark;
(7) fairly good linkage;
(8) big finale at the end; and
(9) PCs have the opportunity to actually have allies!
I think past reviewers are correct in that book two isn't as linked. This was done mainly to let the PCs go up in levels so that they would be ready for the later big fights.
Not a problem for me. I increased the size of the map and added more features and also changed the nefarious end as I felt it was too cliche'.
Overall, this is superlative boxed set. As it isn't terribly pervasive, it can fit in to almost any campaign. For a DM who has little put together, you could do a whole campaign this way but the PCs have to enjoy tromping down in the Undearkr. In my campaign, this started at about 50% of the possible adventures but as more and more happened, it's about 15-20% now. That's sort of b/c now that they're higher level they've gotten involved in big time politics. :)
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