How do you feel about Sigil? Have you DM'd a game that featured it, or been a player in a game where Sigil was featured?
Personally, I dislike the idea of Sigil. D&D lore has detailed a number of cities throughout the planes: the City of Brass, Tu'narath, Zelatar, Hestavar, Dis, etc. Though I haven't done much with it yet (though I plan to soon in my current campaign), the idea that these planar metropolises exist and could trade and have political arrangements or rivalries with one another fascinates me.
I understand the convenience that Sigil presents as a central hub for adventurers to travel to other planes, but I feel that having it as the interplanar metropolis cheapens and distracts from these other cities.