Initially I had reservation with other car rental company in Barcelona and due to their mess up in last minute I switched to Sixt for the very first time and merely based on the reviews...
Reviews turned out to be true, they were amazingly helpful, extremely nice staff and agile process with almost zero wait time. The whole process took about half an hour and they gave me a luxury SUV versus my initial reservation which was standard car. Asked for child seat and it was promptly provided as well.
However there was an issue with my bill which they charged me extra day for returning the car with delay which was not right, so I called the CS and they fixed the bill immediately and agreed to credit the amount.