I think it's unfairly maligned because the tone, atmosphere, setting and encounters are completely different from the 6 modules that went before it. Also considering that years had passed since the last module in the series (D3), and that Gygax was not the author, people really just didn't know what to think about it when it came out.

When I bought it (1981?) I remember thinking that it was strange, but to me it seemed a natural extension of the rules in the DMG. Other worlds and planes were supposed to be mysterious and completely foreign. The DMG had an Arabian City of Brass on fire, and the two examples of other dimensions (tie-ins to TSR games, of course) were Gamma World and the Wild West, with vague hints at Pellucidar, John Carter's Mars and Metamorphosis Alpha. Why should a non-Euclidean extra-dimensional spiderweb be inappropriate?

The module also has some truly fantastic and compelling encounters. The chamber with the 3 drow thrones, the labyrinth of endless spiders and the dwarven city hopelessly besieged by chaotic evil come to mind. The rules for extra-planar play are scary, internally consistent and groundbreaking. But the running encounter with Lolth is a bit weak and the proto-steampunk spidership is not to everyone's taste.

It's not a big deal, of course. BITD we just ended D3 the way Gygax intended - big epic battle below the Fane in the Vault of the Drow. His Q1 was going to be going from the Fane down to the Sunless Sea and engineering a battle between her and the Elder Elemental God, as I recall, and then probably some kind of escape through portal or ancient ruin or whatever, focused on the talisman shapes. Conservative play groups can ignore Q1 and call it good.

/shrug

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