I don't but I expect that it will depend on how much you drive on the French toll roads. 

just copy pasted this from another site: http://www.france-forum-frenchentree.com/viewtopic.php?t=2036

hope it helps.

All the autoroute companies offer Telepeage and they all work on all autoroutes, so it doesn't matter which one you go with.

I got mine online with www.sanef.com

Just fill in the form to register etc. The cost is 30 euros for the transponder (refundable if you take it back to them). You then pay 2 euros per month fee (but only for any month you use it - so only going to France in say May and August will only cost an additional 4 euros).

Buying online you need to give your UK credit card details and they take the total tolls from it each month. If you stop at one of the shops at the toll on a road you can register and use your French debit card for payment. I believe you could call at one of the shops and change your UK credit card payments into French debit card later but I haven't bothered for myself.

I registered online on a Sunday evening and the transponder arrived in the post on Wednesday morning.

I did approach the barrier with great butterflies the very first time I used it but as you get slowly to the barrier and have barely stopped there is a beep and up goes the barrier - very satisfying.

You don't of course get any receipts so you need to keep a note of the useage to check they haven't charged you for anyone else.

You also should remember to remove the transponder from the windscreen when only travelling locally as I would think it would be a target for a car thief if it is visible when you are not there.

It really is so simple and I would not be without it already.

The above is accurate for occasional users, but if you're commuting (i.e. you use exactly the same stretch of motorway many times per month) you can get better offers, e.g. you still pay 2 euros per month but then you get up to 50% off the tolls. See www.atmb.fr for details.