No personal experience, but I know a little about this school, as I know quite a lot of parents who send/have sent their kids there. My dd also knows quite a few kids there. Have name changed, as I don't wish to out myself.
Some parents seem very happy with the school, and their kids have done well. Others have been much less happy, and have chosen to send younger siblings to local state schools instead.
I don't think it's hard to get in at all. Quite a few kids from dd's state primary did the test, and all of them were admitted, including some who were average at best. I have no idea how well they deal with SEN or neurodiversity, I'm afraid.
Teenage dd tells me that there are lots of rumours among young people locally about the school having a bad drug culture. I don't know if this is true. If it is, it doesn't appear to have affected any of the kids I know personally, but we do know of one boy who went badly off the rails after moving to the school - total personality change, and not in the way that his parents were hoping for. That might have happened anyway, of course. I don't think the drug thing (if true) would necessarily affect a quiet, studious type, but perhaps more of a concern to investigate if your child is on the more outgoing "cool" end of the spectrum.
Another thing to be aware of is that there have been allegations of racist bullying at the school, and about such issues being very poorly handled. Again, I don't know the full facts, but that would definitely be a concern for me. You can find out more about this if you Google it, as it was featured in the local press.
As for the kids I know who are actually at the school, most of them seem to be doing well academically, but a lot of them are the type who would have done well anywhere. They don't come across as particularly confident, articulate or cultured - no different from lots of other teenage boys tbh. Personally, I don't really understand what it is that parents are paying for, but as I said, some of them seem happy enough.
I have heard slightly more positive things about the girls high school (or at least, fewer negative things!), though a lot of the girls that dd knows there seem very keen to transfer to the local college in the sixth form. We could have sent dd to the high school, but after researching all the options and visiting the school, talking to other parents etc, we didn't really think it was worth the investment.