Cheaper ... no ... actually a shade more expensive as you'll be charged 1.60 euros per month you use it (ATMB) ... ATMB link
Faster ... yes, anywhere where a queue is formed ... and occasionally that can save a lot of time and frustration.
Also adds a great deal of smugness when you pass that speeding BMW held up in a queue, whilst you sail through the express lane ... of course he'll pass you later ... but the smug moment has been achieved
Don't forget to leave a review of the French and other European campsites you have visited!