It's a curious phenomenon, but I think reading all of these Planescape books might have made me excessively jaded. My knee-jerk reaction to many of the locations in Tales From The Infinite Staircase was "I've already seen this, can't you show me something new?"
But the more I think about it, the more I wonder if maybe it's part of the purpose of an adventure book to put the already established setting material to work. Instead of thinking, "this is a Planescape adventure that takes us to a bunch of familiar locations," I could, in fact, judge it against a "standard" adventure, which might take the players to a forest then a swamp then a lich's tower. Don't think of Tales From The Infinite Staircase like I'm a blogger that's just spent the last eight months reading the bulk of the published supplements, instead think of it as if I were a new player, weaned on the implied setting of the corebook. . .
I think, if you come at it with fresh eyes, this adventure might blow your mind. Pass through a magical doorway and find yourself in the hive-city of the ant-centaurs. A wicked curse has infected these industrious creatures with a magical malaise and now half the town is flooded. Brave these dangers and follow the clues to a bleak world where a dark power lurks in the waters and the natives gain magical powers by allowing worms to burrow into their skin. Then go to Limbo, which, to be a buzzkill, is used in a pretty boring way here (I think, ironically, the fact that the entire plane is pure chaos means that it also feels homogeneous - no location is durable enough to have unique properties). But this lull only lasts for a little while, then you're off to a ruined city on the edge of dreams, a metropolis floating in the clouds, and an infinite library that exists in the space behind a mirror. Wind up in a city made entirely of dangling chains, where you confront the diabolic chain-creatures who just spent the previous seven chapters trying to spread this spiritual plague.
It's an epic adventure through these incredible fantasy worlds, and it's kind of weird that my reaction should be, "Wow, Baator is doing something Lawful Evil? What a surprise. Yawn." Maybe it's the dark side of grid-filling. Assign every conceivable thing a location in your cosmology, and you start to feel a complete lack of shock when a new thing slots perfectly into your system, and thus comes from exactly where you expect it to come from.
Maybe just take the events empirically and enjoy the ride. Don't try to figure out how these locations relate to each other, and forget the cosmology that links them. It's a real irony. This is a characteristically Planescape adventure that uses Planescape as effectively as it's possible to use Planescape, but the weakest part of it is the baggage it inherits from Planescape. Like, it's pretty effective and creepy that the Iron Shadow (the apathy curse that drives the plot) reduces the Rilmani to their essential salts, but when you explain that this happens because the paragons of True Neutrality can't endure being infected by Lawful energy, that . . . takes some of the magic out of it.
Weirdly, I don't think I am alone in this feeling. Tales From The Infinite Staircase sometimes feels like it's holding the broader Planescape setting at arm's length. Part of this is subtle. It largely eschews the cant, and when it does show up, it's usually with a parenthetical explaining what the slang means. But there's also a much larger clue - the titular Infinite Staircase itself. This isn't the first time it's shown up, but as the centerpiece of this adventure, it feels a lot like something that fills the exact same niche as Sigil, but better.
This isn't even me projecting. The text basically says as much, "Once a body's found a place like the Infinite Staircase, she can forget about mucking around with all those sodding portals and their thrice-damned keys." Now, granted, that's an in-character sentiment from one of the margin quotes, but it does persuasively sum up the whole vibe. There's this huge, twisting staircase that branches and folds like Escher-space and every so often there's a landing with a door that connects to a random door in the multiverse, and you can just pop in and out with no problems. All the utility of portals, but much less finicky. Some of the landings are even big enough to house entire villages. You can make the Planeswalkers' Guild your new home base and you never have to go to Sigil again (unless, hilariously, you fail the mirror-library quest and discover that the rare tome you were seeking is also available on the open market).
So what we've got with this adventure is a fairly interesting plot that happens in some fairly interesting places, connected by a framing device made entirely out of redundancy. It really is optimally suited for groups that want to play a Planescape adventure without actually playing a Planescape campaign. It's up to you to decide whether that's a strength or a weakness.
The other interesting thing about this adventure is the way it's structured. In theory, it's possible to visit any of the seven locations in any order. It's unclear what would happen if you went to the chain city first, because the Iron Shadow spreads like a disease and the vectors are already out there, so you can confront the creature that decided on this plan, but you won't have the cure and the damage will be going on in the background, but most of the other locations are fairly interchangeable. Each one has a piece of the puzzle and each chapter begins with a timeline that adjusts the adventure based on things the PCs may or may not have already done (these are extremely boring, but they become less confusing as the adventure goes on).
It's an ambitious design and I'm curious how it worked in practice. The way it's set up, the keepers of the Infinite Staircase give the PCs a list of places where they've detected the Iron Shadow, and the PCs can either work their way down the list or skip around at their pleasure, and I guess that's a species of freedom, but it also feels like a choice that's so arbitrary it might as well not even exist. Which of these seven meaningless names most piques your curiosity? If you pick anything other than the next one numerically, you're going to force the DM to adapt the adventure on the fly. Nonetheless, I admire Tales From The Infinite Staircase for trying something different.
Overall, I'm fairly comfortable pronouncing this adventure "good." There's some great imagery and memorable locations, even if the antagonists tend to lack a significant presence. The real adversary is a magical disease/curse and that's a bit impersonal. If you run it, it might be a good idea to introduce Quimath (the chief of the chain-monsters, who wants to spread the Iron Shadow as far as possible) earlier in the story, so the PCs can put a face to the opposition. And it does sometimes feel like an alternative to Planescape, rather than an introduction to it. But I think there's a solid foundation here.
Ukss Contribuiton: I rather liked Blurophil, the floating city. It harnessed windmill power, in a vaguely medieval technomagic way (the windmills powered the spell that keeps people from accidentally walking off the edge). It had a Festival of Lights that sounds pretty neat. And it was founded by refugees from one of Planescape's few completely original Prime Material worlds (Ortho, the place where the Harmonium successfully "brought peace to the land" by, among other things, ethnically cleansing Blurophil's founders, the Riven).