Objectively, this is bonkers stuff, but I adored it and spent a lot of time thinking about the Outer (and other) Planes, aided no doubt by my fascination with the demons and devils of the Monster Manual. Despite Gygax's precise distinctions between the alignments of the Planes, AD&D didn't have a lot to say about them for a long time, aside from the occasional article in Dragon, like the ones about the Astral Plane (by Roger E. Moore) and the Nine Hells (by Ed Greenwood). And, of course, we later got Gygax's own developed thoughts about the Inner Planes, which were every bit as eccentric and persnickety as what he wrote in Appendix IV of the PHB all those years ago.
What I always wanted was a better sense of the Planes as a place and, more than that, as an adventuring locale. What sorts of adventures could AD&D characters have among the Planes? What made the Outer Planes different from the Prime Material Plane and how would this impact the kinds of adventures to be had there? The better Dragon articles, like those of Greenwood, did this well, or at least better than did Gygax, whose own ideas, while fascinating, remained largely in the realm of the theoretical. I wanted something more "down to earth," if you'll forgive the phrase. Jeff Grubb's Manual of the Planes was a good first step in that direction, but I wanted more.
As it turned out, I'd have to wait until 1994 to get that, in the form of the Planescape Campaign Setting – and it was not at all what I had expected. As imagined by David "Zeb" Cook and brought to visual life by Tony DiTerlizzi, the Outer Planes were indeed weird, though quite different from how they'd been previously portrayed. Instead of being presented as primarily the dwelling places of gods and demons, the Planes were instead a battleground between various factions of "philosophers with clubs," each of which hopes to remake reality according to their own idiosyncratic perspective. These factions, each associated (in some cases loosely) with an alignment or Outer Plane, were the driving force behind Cook's vision for Planescape. More than that, they provided an easy buy-in for player characters looking to involve themselves in the cosmic struggles of the setting.
"The setting." That's important. One of the clever things Cook did with Planescape was that he made the Planes a setting. They weren't just a place you could visit for a brief time; they were a place you could stay. Further, they were a place where even novice characters could stay, not merely high-level ones with access to potent magic. Further still, they were a place with its own native inhabitants and players could easily take up the role of one of them. Planescape gave AD&D's Planes a life of their own, divorced from the Prime Material Plane where most campaigns were set. Planescape made it possible to play entire campaigns where characters never once set foot on the World of Greyhawk, the Forgotten Realms, or any other "normal" campaign world.
This was a bold approach and not at all what I or, I imagine, most AD&D players at the time were expecting. Not everyone warmed to Planescape's vision of the Planes. Indeed, I recall quite a few old hands who scoffed at it as taking too many cues from White Wolf's World of Darkness RPGs, which were very popular at the time. I can certainly appreciate the shock and surprise they probably felt upon reading Planescape and seeing DiTerlizzi's Dr Seuss-like depictions of the denizens of the Planes. This was not Gygax's Planes; it wasn't even Grubb's. It was something quite unique, filled with the strange, the odd, and the occasionally silly, and suffused with a punkish vibe that came through most strongly in its use of Planar Cant drawn from the criminal slang of the 17th, 18th, and 19th centuries. Many people, even fans of the setting, loathed the Cant, but there's no denying that it helped give Planescape a distinct flavor of its own.
Me, I enjoyed Planescape. It was not at all what I expected, but I enjoyed it for what it was: a strange, whimsical, wondrous take on world-hopping fantasy, with "worlds" in this case being other Planes of Existence, each with its own individual rules and style. And then there's Sigil, the City of Doors, located at the very center of the multiverse – if a series of infinite planes can truly be said to have a center. Home to the various planar factions and serving as a crossroads of the Planes, Sigil could serve as the basis for an entire campaign in itself, but it was also the perfect "home base" for planar characters whose adventures took them across the realms of the Great Wheel and beyond. Like Planescape itself, I really enjoyed Sigil and had a lot of fun with it.
I have lots of thoughts I could share about Planescape, both positive and negative, but my overall feeling for it is one of affection. I first made use of the setting as an adjunct to an ongoing Forgotten Realms campaign I ran in the mid-1990s. Later, I ran a "native" campaign among the Planes in the early days of Third Edition. Both were very well received by my players. Indeed, we still occasionally talk about some of the adventures they had in the setting. That's my usual measure of whether a gaming product succeeds – did I have fun with it? – and by that standard, Planescape is one of the greats.