Quickleaf
Legend
I tend to agree (although the bit about demon lords being deities goes back to the 1st ed AD&D DDG - so I don't think Planescape can be blamed for that).
But I do find it more about setting for its own sake, than play. I have a copy of the much-vaunted Dead Gods, and it strikes me personally as unplayable because of the degree of railroading that would be required; likewise Expedition to the Demonweb Pits, which at least presents itself as being in the Planescape style.
For a contrary view, though, I'll call on [MENTION=20323]Quickleaf[/MENTION], who took me to task a couple of years ago when I first expressed an opinion similar to yours!
Unfortunately, the Planescape adventures weren't very good, and the stand-outs were only average. Then again, that's my opinions of most 2e adventures put out by TSR regardless of setting.
As for the playability of the Planescape setting...it depends on the group. Though I am not convinced Planescape has been forced on the core D&D setting, I *do* agree that it should not be treated as core because it is so strange. The sheer strangeness of the setting may have repelled more people than it attracted.
My personal experience was that is was very playable, though. The whole setting was laced with adventure hooks, "planewalkers" were the equivalent of the "adventurer" sub-culture, there were low threat/high threat zones, and travel could be instantaneous (side quest to get portal key) or require the use of a planar pathway (side quest to traverse the River Styx, Yggdrasil, or Infinite Staircase).
I'd argue for its playability, but definitely not for it universality.