I enjoy Planescape as its own unique setting. I would happily play in a Planescape campaign.
As the background for other settings, I find it--and the Great Wheel, if you want to separate them--rather lacking.
Other people have already gone into a lot of reasons I agree with, but in essence it feels artificial to me. Too balanced, too precise. It feels like a game creation, not like something out of myth/occult belief. The 4e World Axis has its own flaws, but I felt it captured the idea of "other planes of existence" in a more organic fashion.
That said, it's important to understand where I'm coming from. I consider the whole "there must be universal balance in all things" philosophy to be such utter nonsensical prattle, I'm sometimes amazed at how many people take it seriously. The idea that good can't exist without evil, or that there can be such a thing as "too much good," is to embrace an absolute misunderstanding of what "good" means. (The Kingpriest on Krynn is
not an example of "too much good," no matter how the books might claim he is. He's an example of a flawed human who did something bad that he
thought was good. And frankly, the gods' response is also downright evil; they could have easily made the same point without killing millions of people for the sin of one man, or one Church.)
So, yeah; not so much a fan of the perfect symmetry.

(Though again, I can't stress enough, taken as its own setting with its own setting-specific quirks, I like Planescape quit a bit.)