I have, so far, had a fantastic experience renting from Sixt, both in the US and in Europe. I have been very happy so far, and as a Platinum member.
I recently became a Diamond member, and my first rental as a Diamond member was at Dubai, and the experience was terrible.
The rental options in ascending order is Toyota Yaris, Nissan Tiida, Mercedes C-Class, Mercedes E-Class, then up from there. I can't find any documentation on what Diamond level comes with, but I thought 1-class was guaranteed and 2-class based on availability?
I booked the Nissan Tiida thinking this would get me into the C-Class. Upon arriving at the station in the parking lot of Terminal 1, the counter agent told me I had been upgraded to a VW Passat. (A category not even listed as an option on the booking site.)
I asked him if I can get an upgrade to the C-Class and he said he doesn't have any available. I asked him if he has any Mercedes available, he said no. I tell him that all of the Mercedes are available for booking online. He said none are available. So, I asked him if I booked the C-Class what would happen? He said that it's impossible to book anything, as it takes 24-hours for them to be notified of a new reservation.
So, since I wanted to C-Class I went ahead and booked it on my phone. He then changed his tune and said I need to go the Sixt counter inside Terminal 1 to process my new reservation.
So, in the 110 degree heat I walk from the car park to the terminal. In the terminal Sixt desk there is a much more friendly lady. She sees my original reservation for the Tiida, and calls the guy up at the car park and asks why I can't be upgraded to the C-Class. Then I hear her, multiple times, say, "but he is a diamond member". Eventually, she gets off the phone and says all she can upgrade me to is the VW Passat. I then told her that I made a booking for a C-Class and wanted to cancel my Tiida reservation. I also ask her what upgrade I should be able to get since I booked a C-Class, due to the diamond status. She calls the guy back and says all he will let me have is a C-Class, with no upgrade whatsoever, since there is nothing further available to upgrade me to.
I tell her that the Sixt website shows an E-Class available (among a plethora of other Mercedes models), what happens if I book it? She says that they will cancel the reservation because they have none. So, I ask her if it is normal that people book cars and that Sixt just cancels it if they don't have it? She said yes.
So, my take away from the DXB location is, based solely on my experience:
1) They have no problem lying to you.
2) They have no interest in following the Sixt elite status program.
3) They don't value your business.
My next stop was in Doha. I rented a Toyota Camry. Upon arrival they told me I was getting a Nissan Altima. I asked them if this is an upgrade and they said no. I then asked them whether they upgraded diamond members. The guy muttered something and said I can have a VW Passat. I asked about a Mercedes and he said they don't have any available. I told him there are many available for booking on the website. He said what the Dubai people said, if I book a Mercedes the reservation will be cancelled since they don't have any.
The VW Passat they gave me had an empty tank of gas and no windshield fluid (the light was on and with the dust all over the place this is somewhat more needed than other places). I also chose the GPS option, they gave me a portable Garmin with a 2-year old map loaded on it, and with all the road changes in Doha the GPS would constantly take me to dead-ends and circuitous routes that kept ending up at crossways that were blocked, so was nearly unusable.
I don't know if I just got very unlucky at both stations or whether this is SOP at the Middle East Sixt locations. I certainly am going to consider other options there next time, as you just end up feeling cheated by the Sixt locations there. Another thing of note is that both cars I got exhibited some really strange engine behavior, accelerating from a stop the engine boggs down then surges. I suspect the air filters are clogged with dust and rarely serviced/cleaned.