It depends on which school you're talking about. I went to a state school, and did very well there. Yet at that same school now, there are many problems which means that parents are looking to take their children away from the school.
I know of a few people who go to independent schools and are having a fantastic experience, but this might not be true in all cases.
I have always been a strong believer that every child will thrive in an environment that is right for them, and it is the job of the parents to work out which environment that is. It is a tricky decision sometimes, but by weighing up the pros and cons of each choice, you should find that you are able to make the right one.