I just came back from one week Paris travel, I rented a mobil wifi device from a company called Hippocket wifi.
Good: savings from roaming, I was able to use iphone 6 with wifi phone call feature, 24/7 broadband wifi access made my travel very convenient.
Bad: the company had originally arranged to have the device picked up from CDG airport, they later notified me that the airport location is no long available and made another arrangement to pick up at a post office near where I stayed. When I attempted to pick up the device from the post office, I discovered the company fail to email me the pick up authorization document. I spent the entire afternoon trying to solve my wifi access issue. Make sure you receive precise pick up instruction from them or you will have to deal with some hassles upon arrival.
What was the cost for this?
86 Euros with unlimited data for about a week, the device allows up to 10 device log in.
Thanks for the information. There are lots of questions posted about this, so it's good to have a report from someone who actually did it.