We had some family vacations with the kid for Christmas, so we rented a Nissan Qashqai (medium SUV) from Avis at Charles De Gaulle airport. I think I have never had such a bad experience with a car rental company despite renting around one time a month in various countries in Europe:

• We had to wait in a queue for one hour to pick-up the car. There was no priority queue for Avis Preferred and only one agent on duty. He was obviously overworked by the sheer number of people having booked a car, some wit complicated requests or paperwork to process
• When came our turn, he asked us how many people we will be in the car (3), and asked whether I was happy to drive an automatic (which is not common in France). I said I was ok but prefer manual. He then proceeds to give us a Nissan Juke, which I didn't know was much smaller than a Qashqai.
• When we go pick up the car, we see we can't fit our things and the baby easily in the car, so I try to go back and get the car we originally booked but get to know "that's all what they have"
• When returning the car, despite this being during the normal opening time for Avis and not during a holiday, there was no agent to process the car, only a security guard who could barely speak French and explained to us we shall give the key downstairs. So I could not get the invoice on the spot
• When the invoice finally came, I got overcharged by around EUR 100 compared to my reservation and I don't even know why, it seems to be car category as we have not been charged for damages or extra distance.

So all in all, instead of being upgraded we have been downgraded in category and ended up paying more + a lot of minor additional inconveniences. I ave written to customer support to complain but also wanted to share the experience here. It's amazing how car rental can get away screwing their customers this way.