My son is at St Bernards, and I have a niece there too. My thoughts are:
There are a lot of traditions, celebrations of feast days, religious services etc. Lots of Catholic values and teaching (obviously!)
The teaching seems to be very good. The academic success is excellent. The sport side is not as good as some. There are no 'after school' activities (because of buses home) - they do a lot of things in lunchtime break. The library is used as a homework area for after school.
There is a fair amount of homework/self directed study. The curriculum is challenging (lots of thinking 'outside the box' and reading around subjects) and really seems to stretch the kids. In my opinion some children would find it difficult to keep up. There are children in my son's class who shouldn't be there. They don't work, don't contribute and behave badly. I guess this is the same for every school....
Children come as far as Chiswick to the East, Maidenhead to the West, Ascot and Bracknell to the South and the Chalfonts to the North. There unlikely to be many 'local' children in your child's class.
It sounds like there will be no free transport to the school provided from September. There are a lot of coaches that come from all the different areas, they do go 'round the houses' a bit so you may have an early start/late arrival home. It's expensive in my view.
Music is good, lots of outside music teachers, good drama and artistic opportunities.
Parents are pretty much kept at arms length - although in my experience this seems to be a common theme with senior schools. You may not meet your child's form teacher until the end of the year.
The community of Nuns left last year, so it is now a 'catholic grammar' rather than a 'convent'. It was a shame as the nuns gave the school a real community feel and some of them were teaching staff.
I understand that this year there were approx 500 applicants for 120 places - but that sounds a bit exaggerated to me. I know there were 200 children on the Saturday exam day.
Traditionally, St B's is the 'catholic one', Herschel is the 'Maths, Sport and Computers' one, Langley is the 'We'll take all the high scorers' one and Slough is the 'languages and all round' one.
Hope this helps, do ask more and I'll do my best to answer.