As stated ages ago, I went to a GS. It wasn't 'super-selective' as such (though it is now- I do wonder if it might be one mentioned earlier that's now full of wealthy parents who 'prefer not to pay'?!),
I know of several girls who left at 16 with 4 or 5 'O' levels. Imho, they shouldn't have been at the GS. We also got quite a few girls from the local girls' SM into our 6th form where you had to get 7 'O's to get a place. Many of them heaved a sigh of relief that they'd finally made up for the shame of failure they'd suffered since 11. One girl left my school in Y8 to go to the SM but no one joined us locally from Y8-11.
I now deliberately live in an area with a 'outstanding' comp in, yes, Hants, like Talkinpeace. This is because although DS1 would definitely have passed my 11+ (but possibly not the new one and I was uneasy about him attending a school that now has significant numbers of boys who holiday in the Maldives) I knew he'd be fine and happy at the comp we 'chose'; but the real reason we went with this school is that DS2 wouldn't have passed an 11+. He would have suffered the lifelong burden of 'failure' at 11 and could find himself in classes that contained not only the DC who came 24th out of 100, but the one who came 80th as well (as happened in my brother's SM), if we assume 1st to 23rd are at the GS.
So they will both be at a comp where both will be taught in ability bands; some lessons will always be taught in all ability groups i.e all parties will know of the existence of the others and interact with them on a daily basis.
The main reason I'd favour a GS, if I did, is that there is only a small slice of MC life represented; that's to say there's a lot less indiscipline and low level disruption because a) in theory DCs can be selected out as well as in and b) the DC should in theory be clever enough to recognise that to shut up and listen is in their own interest (and c) the lessons move at an appropriate pace for the DC's intelligence- I could add I had many teachers at my school who'd've been eaten alive at a 'normal' comp!).
Well, guess what? There's little low level disruption at my DCs comp and they are pretty much taught academia in ability groups. BUT should DS1, for instance, want to get heavily into performing arts, or woodwork, for instance, he can use first class facilities because the school take this stuff as seriously as triple science and 3rd MFLs. This school easily outcompetes a lot of Kent GSs, I also note, in terms of league tables!
My final thought is that the problem with today's GS are that they are divisive, they ensure the privilege of the few over the sense of failure in the others; they can be very hard to access by DC who maybe bright but who haven't had the advantages of private prep schools, for instance. Maybe there is a place for 'special schools' for the preternaturally bright, the 2% but scything off the 'top' 23% is blatantly unfair and socially divisive.