Outside of certain limited situations where French immersion is handled very well because it is true immersion and the teaching is strong, it has failed the people I know.   The people I know who speak the most `natural` French notwithstanding coming from homes where English was the primary language all learned it in true immersion situations.   (e.g., most friends French, attending a French-only school, attending summer camp where the language is French for French-speaking kids.)