I think state/ private choice is such a personal choice...(and quite a good way of sparking off impassioned debate on MN!).
Bottom line for us is that we can't afford private...ever. But if we could, and being totally honest I would only send them private for secondary. Not because I think the education is better/ want them to go to Oxbridge etc, but I love all the extra curricular activities on offer on site - with 3 kids, there are only so many places I can be at once as a 'soccer mom'.
Just in my own experience (and someone else will feel very differently to me) as mum, trained teacher and childminder with 2 kids currently in primary: I think a large part of the Infants experience (Years R, 1 & 2) is about socialising, learning how to deal with peers/ teachers in school environment, and how to enjoy the learning experience. But have to admit, it is an 'Outstanding' state primary, and our experience of admin/ teaching/ differentiation of work has been great.
Friends who went private have a lot more Homework, spellings, reading books changed more often... so when I could not withstand the panic of my children being 'left behind' by those whose parents could pay for education, I just did a bit more with DC myself at home, 5-10 min of spellings or maths a couple times a week...
Esp living in London, where schools, homework, extra curricular activities sometimes feels incredibly competitive and hot-housey, I really wanted my children to just enjoy school, particularly the first few years.
I think you should think about what works best for your family, and then ignore everyone else's opinions. In the nicest possible way. Sometimes, I think we rubbish other people's choices in order to validate our own choices - or lack thereof. Just because I can't afford private, I try to not be bitter :)