SPSJ is going very well for ours (last year before senior school). The head has changed but I've no interaction with her.
The school has, as much as is possible in an expensive west London school, a down to earth feel. It's hard to pin down what that really means.
They were very good at introducing the new boys to the existing peer group at 11+. The state school boys did a number of Saturday classes on French and Latin before they arrived. Whilst this sound onerous, actually it meant that they all had solid first friends before they arrived in September and were very familiar with the classrooms and some of their teachers. On top of that they organised for each new boy to have a buddy from the existing year. Our son's buddy was great and thoughtful. The parents too were offered numerous opportunities to meet each other and existing parents and teachers before September. To be honest, the whole process was like being wrapped up in cotton wool. It may be too much for some!
The thing that impresses me most about the school is the, for the most part, youthful and energetic nature of the teachers. My son has taken to subjects I would never have dreamed of because of his respect and liking of subject teachers.
The downsides are much the same as for any fee paying school. There are some parents whose wealth is, perhaps, a hinderance, but boys are very good at selecting their friendships and we, so far get on very well with parents of his chosen friends. I also wonder if there are one or two parents who wear the school a bit like a badge of honour.
In the longer run, I'm impressed by the Head at the senior school on the few occasions we've met etc. Finally, by the time yours would get there, the £70m+ renovation will be complete and it is somewhat amazing set in 40+ acres by the river!
All this said, I know nothing of KCS, so can't offer any comparison at all.