This world is likely to be a lot like our friendly Spelljammer; incomplete, and untended. This world has NOT been revised, there are no (significant) novels, there is little talk of it in the other sourcebooks, and it sounds inplausable. I do have friends who enjoy it, but I have not been able to see any of their stats, or hear anything positive (other than the general comment of it being good).
Sigil is a city on the edge of the planes of existance. It is run by the keeper of the city, with the powers to restrict powerful beings. Gee, if your not on the Prime Material Plane, then you'd probably bump into a god or simmilarly powerful being. What are you gonna do now, pull out that +9 holy avenger that won't even tickle it?
There are only one or two novels in this world (if any), and I have yet to even see them.
This is the only Planescape site on the net, and it isn't too great. Oh, well.