Amsterdam Schiphol
SIXT Platinum
Booked: BMW 5 series / Jaguar XF or equivalent (LDAR)
Allocated: BMW 5 series (LDAR)

Received: Was offered Jaguar XF, Volvo S90 or Mercedes E class, but declined and got a brand-spanking new BMW 520i (2021 model) with only 10km on the odometer. Paid €140 for 4 full days.
Friendly lady at the counter indicated that I should book a 3 series next time, as she'd be able to upgrade me to a 5 series for free (subject to availability ofcourse).

The 'upgrades' from a 5 series are Jaguar XF, Volvo S90 or Mercedes E class and aren't necessarily upgrades.

Always nice to get a virgin car. But a 520i (automatic) is a PDAR class, so I think you actually got a downgrade Luxury to Premium from LDAR (530e or Jag XF). Nonetheless I probaly would have made the same choice as you.

Must say that for the 140€ it is ofcourse a very nice deal, with any of the above cars.
i frequently make use of their weekend rate for premium cars.
And the staff there are very helpful to accommodate to ones wishes.

Rotterdam Hilton
SIXT Gold (status match, this is my first rental with Sixt)
Booked: Standard SUV (SFMR) - e.g. Opel Grandland X
Received: BMW 520 Estate (think it's a PWAR)

They called me this morning to let me know they didn't have the class I had booked but that they would upgrade me to a premium car. I told them I'm okay with that but that I need lots of space in the back as I need to transport something big. The guy before me drove off with a Volvo XC40 T3, which left a XC90 as the only SUV parked out front. I was glad they didn't give me that as my excess policy wouldn't cover the VC90. There was some unlisted damage on the BMW but all staff were very friendly and they added the notes. So far so good on my first experience with them.

NYC
Sixt Card
Booked: XCAR (Cadillac CT6)
Assigned: Volvo S90
Recieved: Volvo V60 Cross Country

Not an upgrade, but thought I'd mention it here that if you are renting from NYC and you book PCAR/LCAR/XCAR, you are almost certainly going to get a Volvo of some sort.

SIXT Miami today. SIXT Is usually really good about assigning a car on the app (and then making you wait on the line anyway) but today i got nothing. No line at the counter tho so I walked up and asked what I'd be getting for my midsize SUV and was assigned a Tiguan. But as I was standing there, my app suddenly offered me an upgrade to a brand new Atlas, which I took. But then the guy's giving me the Tiguan and the app giving me the Atlas caused quite a commotion in their system.

That said, I rolled out of there with a very well equipped Atlas (leather, Sirius, 4000 miles). The lot was chock full of Dodge Chargers and every Volvo you possibly imagine.

AMSterdam Schiphol Airport
Sixt Platinum
Booked: LTAR (Bmw 4 serie or Audi A5 cabrio)
assigned; Audi A5 cabrio
received; brand new A5 Cabrio

Nice experience at Amsterdam airport. Took last minute advantage of their fire sale “weekend rate”. Received a brand new cabrio; Audi A5 S-line with 19” rims, leather seats and MMI infotainment system. Rental contract states a retail price of €71.750,- for the vehicle.
No upgrade available due to no higher-end cabrio’s available. They tried to up-sell me into a bmw 8 serie grand coupe, but I was not interested since I already drove that one and this was a sunny weekend.

AMSterdam Schiphol Airport
Sixt Platinum
Booked: LTAR (Bmw 4 serie or Audi A5 cabrio)
assigned; Audi A5 cabrio
received; brand new A5 Cabrio

Nice experience at Amsterdam airport. Took last minute advantage of their fire sale “weekend rate”. Received a brand new cabrio; Audi A5 S-line with 19” rims, leather seats and MMI infotainment system. Rental contract states a retail price of €71.750,- for the vehicle.
No upgrade available due to no higher-end cabrio’s available. They tried to up-sell me into a bmw 8 serie grand coupe, but I was not interested since I already drove that one and this was a sunny weekend.

Downtown NYC Location
Booked: small premium (BMW 3 series)
Received: Volvo v90

Technically an upgrade, but given the sheer number of (FL-plated) Volvos that populate SIXT inventory in NYC, it didn't really feel like it, and would have preferred a sportier smaller car. App assigned me nothing because they were "totally sold out" but when grabbing my car, I saw the lot was totally full.

SIXT Gold
Faro Airport (Portugal)
Booked: IDMR small premium (BMW 1 series)
Received: MERCEDES BENZ A160 (a bit underpowered but new car with only 20KM on the clock)

On return, they have found a very tiny mark where someone has opened the door into this car. Have been charged 470Euro for it, so currently claiming through my own insurer.

Sixt Diamond
TLS airport France
Booked FDAR: (Mercedes C-class)
Received: BMW X3 30D

given a broad choice of cars (mercedes C, Audi Q5, Volvo XC 60,…) took the X3 as I prefer BMW and it had a 3 litre engine
nice car with lots of options on it

SIXT GOLD
NEWARK AIRPORT
Booked: SMALL LUXURY SEDAN (Mercedes A Class)
ASSIGNED: see below
RECEIVED: 2021 Infiniti Q50 with 4k miles

The SIXT app is by far the worst of em. In the past its been totally useless, but tonight it let me choose a car- but only offered me "upgrades" for considerably higher fees. These "upgrades" included a Subaru Outback and Dodge Durango, in addition to BMW sedans/SUVs and Suburbans/Tahoes. It didnt offer me any cars at all to select without a paid upgrade- until i refreshed repeatedly, and then was offered free upgrades to.... the same Subaru Outback or Dodge Durango. Do people fall for this?

And of course you have to go to the desk regardless, and it was a packed room with a long line, and i was offered the same upgrades on the app (for the same fees) before finally getting my new Infiniti. Pretty well equipped, AWD, low mileage so overall not bad considering.

SIXT GOLD
MAINZ GERMANY, CITY LOCATION
Booked: SMALL LUXURY SEDAN (Audi A4 or similar)
ASSIGNED: nothing via APP
RECEIVED: 2021 AUDI Q3, 4k kms

Very easy pickup with very nice staff. Of course was offered an A6 for "just a few more Euro" but was fine with the q3. I expected it to be a pretty base model based on US reports but it was well equipped and a nice ride.

Sixt Diamond
Reims Train Station
Booked CDAR: (Pugeot 308 Auto)
Received: BMW 530i M Sport

Location was very quiet. No choices, but the agent seemed very excited to give me this upgrade. Definitely a nicer vehicle than I had reserved.

Sixt Platinum (Status match)
MIA (Miami Airport)
Booked XFAR: (BMW X7)
Received: 2021 Land Rover Range Rover (Trim: P400 HSE)

This was my first time renting with Sixt. Pleasantly surprised with the upgrade here. Was originally offered a QX80 but then offered an upgrade due to Platinum status.

Bristol Airport - SIXT Platinum through BWR status match
Booked: Sedan (IDMR) - e.g. Skoda Octavia
Received: Peugeot 2008 (CFMR)

Upon arrival at the desk, I was offered an Audi A1 or Peugeot 3008 both for 50€ extra, or a 2008.
Asked for the Platinum upgrade but was refused. Emailed the loyalty program who confirmed I am entitled to the upgrade.

Took the 2008 not realising it was a downgrade (awful car by the way) and it was the only vehicle offered that didn't require extra payment. Looked at what category the 2008 was in and was having issues with the android auto upon arrival to my destination.

Ended up phoning the station to complain about being downgraded to a smaller car and was offered to switch the car the following day to either, a C5 Aircross auto (IFAR) or a BMW 1 Series Auto (ILAR) after the agent tried to convince my the CFMR was an upgrade to the IDMR... Took the 1 Series
Not the best experience overall.