A few years ago I rented from Avis which was located in the Hotel Marques de Pombal where I was staying. It was convenient and no problems.
I have rented in France from Sixt and had no problems at all. The counter was closed and I had to leave the car in the parking lot at CDG and drop the keys in a box. I was worried. Took tons of pictures of the car and the key being dropped etc.... expecting problems but never had an issue.