Sean over at Power Score did an excellent rundown on the Factions of Planescape, organizing all of the information about each from a large variety of canonical sources:
Dungeons & Dragons - A Guide to the Factions of Planescape
It's up to his usual standards of organizing information from different sources, and it's probably useful if you run Planescape. Me, I really like Planescape.
Planescape has such a "let's get those people who like Vampire: The Masquerade!" feel to it, though. The art, the very affected in-character-ish way the books are written, the typeface, the pastel colors and spiky bits everywhere. Even the factions have that Vampire feel - each has a name, cool powers, a nickname, an attitude that comes with it, etc.
But the concept of Planescape is very strong - belief affects everything. Belief defines everything, and the beliefs of enough people can shift planes and affect the world strong around them. That and the bit melting pot mix of Sigil - devils and demons rubbing shoulders with good-aligned adventurers and an explanation for every oddball mix - is a great adventuring base. Planescape: Torment does a good job of setting the scene and feel of the setting. So I'm always interested in reading more about it. That post highlights some the very play-driving detail that is out there for Planescape.