Planescape came out at the time I was taking a hiatus from D&D because second edition had turned me off, and I'd migrated to playing GURPS and other RPGs. During this period David "Zeb" Cook took the concepts of Outer Planes from the 1st edition Dungeon Masters Guide and did what the Manual of the Planes should have: turned it into one of the greatest settings of all time.
I didn't discover this until years later when I picked up the Planescape boxed set at a garage sale around the time that Planescape Torment, a computer roleplaying game, came out. So it must have been around 2000. Torment was one of the best CRPGs in its day.
I haven't played the Planescape setting at the gaming table as much as I'd like. A few short sessions, but I never really got a gaming group that wanted to play a campaign. However I have enjoyed reading those books, and will always have fond memories of playing Torment. Also, a lot of the art and design is really interesting and different.
Legend goes that the symbol for Planescape: a female face surrounded by blades called "the Lady of Pain" is a representation of Lorraine Williams. Because everyone loved working with her so much?
One of the most compelling concepts in Planescape is the city of Sigil. This city was set in the middle of the Concordant Opposition and was a meeting place for all kinds of extra-planar beings. What made this so compelling was that the city was full of doors that lead to other planes. You can see from a gaming perspective what an awesome idea that is, as well as a logical feature that fits in nicely with the entire idea of the setting. Too bad Planescape hadn't come into being much sooner, well if Manual of the Plans had been this we'd have a lot more fond memories of that book and the setting may have become as large as Forgotten Realms. But, by the time it came out there were a lot of distractions for the gaming market with everything from dozens of popular role-playing games, computer games were on the rise, as well as the release of Magic: the Gathering which changed everything.
On the Russian blog Nether-Whisper there is an interactive map of Sigil that is very well done. You can explore the city in detail. If you're playing a Planescape campaign I could see this as a valuable tool to enhance the game.