Planescape was my favorite Dungeons and Dragons setting of all-time.
I think admittedly, it was a little more “advanced” than say the Forgotten Realms. What I mean is, if you were playing D&D for the first time with someone new, sitting down and explaining Planescape was perhaps a bit harder, and it meant perhaps a bit more to people that had already played for years with the planes as a backdrop to magical adventures.
But, to contradict myself a bit, the flip side of that is Planescape definitely appeals to a number of core fantasy desires and concepts: hopping into a glowing portal and ending up in a different place is a quintessential fantasy moment. And even someone who has never read a fantasy book or played D&D will probably have a concept of heaven and hell, and know a bit about Greek myth for example - in these ways I think its possible Planescape would be even more digestible then say the Forgotten Realms to some new users.
Either way, the setting had an incredibly strong sense of place for me. That was due to both great writing and fantastic illustrations, in addition to the great core concepts. The illustrations by Tony DiTerlizzi remain some of my favorite to this day.
Also a little contrary to my previous posts, I think Planescape accomplished some of the goals I was bemoaning previously, that Wizards of the Coast brought to Dungeons and Dragons - the push towards fantasy settings that more closely resemble video games like World of Warcraft. It would seem WotC agrees with that, since the Tiefling, first introduced in Planescape, has become a more core part of the game and other settings (and indeed, strikes me as a character that would be right at home in a World of Warcraft setting).
I am surprised it was not more popular at the time of its release, though judging popularity is a relative thing. However, I do think it was far ahead of its time. It was first published in 1994, long before World of Warcraft or the current popularity of concepts like steampunk. However, a (very cursory) ready of current D&D forums online shows its not the most popular setting even now.
That is ok, I am not interested in a popularity contest, I just think the setting deserves more praise. My ideal game is a AD&D 2nd Edition romp set in Planescape.