If this had been around when I was a kid, I think it would've had a huge impact on me. These kids could've been my friends. I knew some version of all these characters when I was in school, though I was a band geek instead of a drama geek. I would've totally been Leah. I still kind of am.(show spoiler)
Now, on the closer side to 40, I find it more quasi-nostalgic, like going back to high school in a gay Norman Rockwell painting - something safely observed from the distance of time and rose-tinted glasses, but nothing I'd want to visit again. Not that my high school days were bad, necessarily, but... it was high school. And nowadays, kids have to go to school with social media. No thank you!
I know some people avoid YA because of the teen angst. Which I get. I'm anti-angst in general as well. This isn't what I would call angst, but there is some teen drama. But here's the thing about teen drama - it's a lot like grown-up drama, only kids at least have the excuse of being kids. They're figuring out themselves and each other and how to have relationships beyond just friendships. And changing in front of people you've known your whole life can be pretty scary at any age. This book captures all that and still stays hopeful. Even when Simon's going through hell of his forced outing, Ms. Albertalli writes in a way that you know everything will turn out okay in the end. And it does. It's a syrupy overload of sugary goodness.
Is it ground-breaking? No. Is it perfect? No. I have some niggles, but not many and not any that would detract my enjoyment of it. Would I recommend it to others? Hell yes. Even people who don't like YA? I'd say at least try the first few chapters and see what you think.