My dd2 went to a comp (not our nearest) last September after being HE'd up to age 13. It's a good school, they're very responsive whenever I've contacted them, she's doing well and will no doubt get good GCSE results in a couple of years. It's been good for her to be near the top of the class, but she really struggles with the behaviour of some of the kids (not an HE thing, just her, she was like it at nursery, and ever since) and has felt picked on at times. She definitely feels that dd1, at a grammar, has an easier time getting an education because she can just get on and work.
So in that respect, I'd rather she was at the grammar school, but she was and is adamant that she didn't want to go to a school where everyone was clever and she wouldn't get any credit for doing well, or to an all girls school.
She was recommending to 9 yo dd3 to try to get into the grammar the other day though.