Well I don't have kids at independent school but I did go to Watson from p4 to the end 
I would say that the Independent schools have AMAZING facilities/clubs/extra-curricular activities on offer PLUS they do produce good exam results. A major reason for this that I noticed while I was there is that they teach you how to study and really start preparing you for exams way before the state schools do (I had lots of friends in state schools, some of whom did really well but I definitely noticed that).
HOWEVER, consider your dc's characters well before you choose. My parents chose private for mainly academic reasons and because they thought private schools were a 'nicer' environment, I think
Well I think kids can be pretty much the same wherever they are. I was quiet and very shy at school and I was bright, quite artistic and fairly sporty. But in Watsons, a school of over 2000 pupils, many of whom are exceptionally talented I was very much middle of the road and didn't stand out in any way at all, as a result I think I actually underperformed!! lol. Plus I didn't make a lot of friends as it was really cliquey right from the start and I was just too quiet to be significant.
I didn't hate school at all. I had some friends and I did well but I don't think it was the right choice and I don't think for me it was worth all the money my parents sacrificed- for us it was independent school instead of 2nd car, holidays, cool clothes, meals out etc.
It really depends on your dc. My ds for example is really outgoing and sociable and very streetwise, I think he would thrive in that environment 
If I had the money and could choose an independent school for him I think I would be tempted to have a good look at Clifton Hall- it's like a prep school but looks lovely.
Oh and Watsons doesn't have really long days. In fact I'm sure the lower primary used to finish at 2.30pm but I could be wrong. My ds started p1 this year at state school and went straight into 3.15 finishes.
In secondary we finished at 3.35 I think but we got a 40min afternoon break at like 2pm or something. Plus much longer holidays. Also Watson's operates a 7day timetable not Mon-Fri
I'll explain if you want to know more!
Sorry for going on a bit!