I’ve only had good experiences with Sixt T1. I’m renting every month almost and consistently get upgrades and good cars. I admit waiting times at peaks are annoying but that’s not Sixt fault, it’s the airport and how much location they provide — it’s a 2-people booth only. When you look at Sixt stations around the world they are huge and well staffed, and as soon as Dubai airport refurbished the rental car location I’m sure it’ll be far bigger and faster.
The cars have always been well maintained, OK some have high mileage, so what?? Does it make a difference in your driving? Or do you just want a new-car feel? For those I recommend the higher class models which are less used... all in all always a good experience here.
The team is super nice and accommodating and patient with customers, even at peak night times. When I get to the outside area, the colleague is usually already waiting for me with the key in his hand.