They don't work that well at all. I've only used them in Florida, essentially at 10am you want a pass for X ride, you get issued it sometimes for 2pm . You can't then get a pass for any other ride till that one expires so you have to carefully pick what you go on in between. I'm not sure what the queue times are in Paris but its sometimes 1.5 hours so you could only go on 2 rides in between then 1/2 hour just killing time.
They do one for the universal parks that work better (not that this helps you!) you get one go on each of the big rides and guaranteed no longer than a 15 min wait. Costs a bit though!!