Start with the obvious: Sphinx is a weird book. If you're picking it up, you likely know that. Whether you know its underlying concept or not, you can probably figure out just from the Oulipo tag it carries that this is going to be a strange and... unique novel.
I didn't buy Sphinx for the Oulipo aspect, to be honest. I purchased the book for another tag: "A modern classic of experimental, feminist, and LGBT/queer literature". One of my goals this year was to read more literature by queer women, or about queer topics. Sphinx isn't exactly what I was expecting, but its Oulipo-style experimentation makes it an interesting statement on gender and identity regardless.
Warning: the remainder of this post describes in part the Oulipean quality of the novel. If you'd rather come in blind, I would advise against continuing this post (as well as avoiding the introduction and the back-cover blurb or really any other review of the book...).
This isn't a real review of Sphinx. You can find far more nuanced and intelligent reviews elsewhere. My experience reading Sphinx was very much colored by my expectation of the queer aspect - the gender-bending, gender nonconforming aspects that were supposed to make the book stand out (specifically the fact that neither the narrator nor their lover are ever given a specific gender). I read the book constantly trying to figure out what my gender default would have been, trying to figure out what my sexuality default was becoming, constantly trying to better understand my biases as regards identity and sexuality. This made the rest of the reading experience feel... tame.
Because crisp as the writing may be, there's not that much here that I haven't read elsewhere. Certainly not the musings on love or the disaffected youth aspect or the glitzy night-life angle. The narrator felt dully familiar, with that false cleverness that often trips me up on books. It was tedious at times, and beyond the constant game of gender expectations, I'm not sure how much Sphinx really challenges anything at all. Because the strength of Sphinx is in its concept (the vagueness, the nondescript, effectively); it doesn't actually tackle very many queer issues. They're there - tangent to the story, hovering around the edges in implications and suggestions - but not part of the story's core. There's not much in the story's core, for that matter.
Maybe I'm just a crank. Maybe I'm not sophisticated enough for "experimental" literature (and I suspect this plays some role). But I found Sphinx to be... alright. Not much more. Aspects were good - I liked the ending and I did appreciate what Garréta was attempting with gender - but on the whole I read the book with a hint of disinterest. Then again, most other readers have agreed that Sphinx is a unique and important book so maybe I'm alone in this. So I suppose what's left is... read it yourself?