I played in a short campaign there. The DM was super into it and knew it by heart from way back. I got pretty into it and did a fair amount of research outside of game as my character was supposed to be a local. The other two players... seemed pretty lost a fair amount of the time. They seemed to have a good time, but the setting seemed more a barrier to entry and a source of engagement. Admittedly, given that I just used "seemed" three times in two sentences, I don't really know that that was the case with them, but it is my impression of how they interacted with the setting.
Its a detailed and original setting, which is great for people who are fascinated by settings and their dynamics, but if you've got players who don't want to have to learn and remember a lot of weird setting trivia its going to be hard for them to know what they can do in this "you can do anything" game. Even if they are super into it, unless you are amazing at summarizing it they are basically going to have to read up in their own time to not be confused endlessly. That works for some people, it doesn't work for others.
As a DM I personally like a baseline of fairly tropey, bog-standard fantasy medievalness in my settings precisely because everyone has seen it before so when I do an imperfect job of describing it people can still imagine what its supposed to look like and what they can do in it. I'm a firm believer that in ttrpgs cliché can be your friend, as it gives people touchstones in a game where a game master has to impart all sensory information to the players. Sigil hates cliché, and wants to be as original as possible with everything, and more power to it for that. But it also makes it as setting that works best when players want to sink their teeth into it, keep notes, and read up about it and generally think about it outside of the sessions, which just doesn't fit everyone's life or level of interest.