NO. Do NOT book with them. We booked a car through CarFlexi with National, pick up in Paros, and drop-off in Chania (Crete) 3 weeks later. Everything was confirmed, we had the agreement. When we showed up in Paros to pick up the car, we were told the booking isn't possible, we can't take the car to Crete.
The only thing CarFlexi was willing to do, is offered us new rentals at twice the price (at least). Our whole trip, including our flights, was booked around this one-way car rental. They didn't admit any mistake or do anything to help us. We're now in Paros, without any car rental coming up in Crete unless we want to fork over an additional 500EUR.
DO NOT BOOK WITH THEM. If anything goes wrong they have NO responsibility to you and won't do anything to make things right.