I haven't stayed there, but I almost did, when I could hardly find a place in Paris.. and my French co-workers told me that's the best budget bet. A couple of them swear by Formule 1.
The crappy part about them is that they're generally off in the suburbs. Still within reach of transportation, but since they're in business/industrial areas, there's nothing around.
That said, they're IMMACULATELY clean (almost to the point of sterility), very secure (all access in & out is via card), and as you've pointed out, dirt cheap. Plus, they're part of Accor, which is a very reputable hotel chain.
Still, I'd pony up the extra few bucks and put up with less clean conditions to get the social atmosphere of a hostel..