I'm still quite young (finishing high school), so what I'm going to say will be mostly focused on how it affected me growing up and in my education.
Much like the current top comment, I got diagnosed early (about 3), because I just came into kindergarten and did lots of things that aren't normal when you're three - I could read and write and do most basic math, even knew some English (I'm French). My parents and teachers realised something was up, I got tested and ended up being put ahead by two grades. According to my parents, the teachers said that I had the competences to go up to 1st grade (if not more) by that time but were too worried that I just wouldn't be capable of adjusting.
Socially speaking, I don't think it helped. I'm not sure if I can attribute it to any smartness of mine though - if I ever developed a superiority complex it was beat out of me during elementary school. Being two years younger, I'd get bullied by the bigger kids. I got quiet. I had no friends - who wants to be friends with the weird kid, the "freak" ? To this day, I'm stressed in most social situations, and there are only a few people in my life I will be able to text first without an infinite worry that I'm bothering them and they don't want to hear from me. Girlfriends are out of the question : who wants to date someone two years younger ? Maybe, if I stayed with kids my age things would have gone smoother ; maybe there's just something about me that would have made it go wrong : I don't know. I probably never will. Good thing is, it got better : I've got a stable set of friends I'm probably going to stay close to for the following years, can sort of enjoy myself at most parties and I managed to become normal enough to fit in. Hell, people give me the "you're mature for your age" speech now - I nod, but honestly : that's bullshit, I just spent over a decade learning to fake hard enough, that's it.
Academically speaking, everything's was a breeze in elementary/middle school. "You're so smart", I was told. I wouldn't do homework, because I could do it on the fly if I got asked the answers ; I wouldn't study, because I'd already understood. I simply never worked. Things were, and in many cases still are so easy for me, but you shouldn't entirely envy me for it. If there's one thing I never learned, it's how to work. As it turns out, talent only carries you so far, and work is obviously and deservedly incredibly valued. What does being the "You're such a genius" kid get you ? It gets you someone who isn't capable of achieving anything that requires actual hard work. It gets you someone who expects everything to come easily. That's not healthy. When I was 10, I wrote that people were like cars : I was given a head-start on a downward slope, while many others started slow on tortuous roads. At some point I'd slow down and stop, and be confused because it didn't go like it always did ; those who encountered trouble would easily get past me, as their harder situation taught them how to become better drivers. I'm worried I'll hit a wall, and I know I'm getting closer and closer to it.
Now I'm not going to go on a tirade saying that being in the top whatever percentile of IQ (whatever it means anyway) is an absolute curse and ruined my whole life. I'm also not going to blame my surroundings for not guiding me towards actually being capable of working. I've been incredibly lucky to have a family who understood I wasn't suited for the standard way of doing things, who allowed me to get ahead and stay stimulated, and lived in a household wealthy enough to get me into a private school with classes adapted to people like me. I've had a good run so far. Next year I'll be in one of the best places in the region to prepare myself for what I dreamt to become since I was a kid, and I can't be sure I would have gotten there had I been the standard student.
Did that whole ordeal bring me some problems most people wouldn't face ? Sure. Did it save me from some problems most people do face ? Yes also. If given the chance, would I start again with 40 IQ points fewer ? Never. It's been a weird life compared to many, but I'm not sure anyone is normal anyway.