Just a small clarification:
Obviously prestige is very important insofar as prestige correlates with funding, networking, and ultimately job placement. What I meant to say is that, judging from what I've heard from professors and graduate students, there are highly prestigious French PhD programs at universities that, on the whole, are not generally considered to be among the most prestigious. NYU, Penn State, CUNY, U of Wisconsin-Madison are great universities, but I don't think they have the same luster as, say, Cornell. However, my understanding is that their French PhD programs are all considered to be at the top in terms of prestige.
Obviously, everyone's definition of prestige will be different, since we are dealing with a rather imprecise, vague quality. This is why I would like to hear different peoples opinions.