Hi Maddy,
I don't have any tips, but I would be pleased to hear of your experiences. Like many others, I suspect, we have looked at the empty Liber-T barrier with some envy. I have lots of questions, like how easy was it to get? Do you need a French bank account, or can you do a DD payment from the UK? Is it worth getting one for 2 weeks' main holiday a year, plus the odd extra week some years?
What happens if you change your car? Does it cover you when towing (I know the tolls are different when towing)? And anything else!
I always use a credit card when going through tolls (except the odd one which will only take cash, of course), but this doesn't stop the queues.
Jo