Location: BCN airport
Status: Diamond
Booked: CCMR (A1 or similar)
Received: CLMR (MINI Countryman)
Duration: 3 days
Driven: 400 km
Comments: Ugh... what a car. I didn't mind the manual or the diesel, but the location of the speedo is just stupid! Also was a pain to park in the small underground spaces a few times.
Other: There is a longer story attached to this and I was not happy with the rental. Sixt CS made good (rate adjustment) after a few emails.

Pick-up: WAW
Drop-off: Lodz Widzew Train Station
Status: Diamond
Booked: EDMR (Renault Clio or similar)
Received: ILMR (Volvo V40, 24K)
Duration: 7 days
Driven: 1000 km
Comments: A great car for the Polish roads. Small but comfortable, good suspension, great gearbox.
Other: Had to wait an hour at arrival because they didn't have it ready even though it was confirmed by writing in the email. Offered a Renault or some French car, declined. Normally I wouldn't bother but for a 7 day rental it was worth it especially with the km driven.
Drop-off was exceptionally easy, I connected with the station (Meet&Greet) and told them I was staying at a hotel. They said that I should park at the hotel and they'll come pick up the car. Seamless!

Pick-up: MCO
Drop-off: Miami downtown location
Status: Platinum
Booked: FCAR (Chrylser 300 etc.)
Received: PFAR (BMW X1); also offered Volvo S60/PCAR
Duration: 1 day
Driven: 240 miles
Comments: Fairly efficient and painless at both locations. Shuttle from MCO to Sixt was pretty quick; drop off at Miami was a little confusing, no signs indicating where to park at the downtown location, but a phone call solved that. Enjoyed the Beemer, although it's been driven a hard 21K miles and certainly wasn't as clean as I expected (could have used a wash, interior needing vacuuming and honestly some carpet stains needed attention.) Oh yes - rear windshield wiper blade was missing, which seemed a bit random. Overall a good experience!

Pick-up: LAX
Same drop-off
Status: Platinum
Booked: PCAR (Merc CLA)
Received: Choice between Volvo V60 and Audi A5. Took the brand-new (1K miles) A5 (S-line Quattro but with literally ZERO options, not even a USB port for my phone), next day traded it for a BMW 328i (27K miles and a bit beat up, but enjoyed driving it so much more; did have a USB port and, of course, I didn't use it once). Was also offered a Merc C300 but saw the BMW and figured I'd rather drive than chauffeur myself around.
Duration: 3 days but returned 1 day earlier; still got charged the same, so this ought to be fun (I pre-paid; when I had returned cars without a pre-pay before, I always got the unused day removed)
Comments: Sixt LAX is what it is - a fairly efficient process when there aren't tons of people around and generally a fairly limited selection of cars, as well as a semi-convenient place to get to (basically on par with the other rental companies at LAX as it is offsite and requires a shuttle which is hit-and-miss timing-wise; the parking structure with multiple REALLY tall speed bumps annoys me greatly). PCAR looked absolutely barren - I didn't see a single CLA, A3 or 228i. Was curiously offered a Corvette as being in the same class as the A5 (yeah, right), but then was told that it'd cost extra (which made sense, but saying it was in the same class as the Audi didn't). As I'm getting used to, the chances of getting a well-optioned car are low: not one of my LAX rentals had Nav and the BMW didn't even have an active Sirius subscription (the A5 did). Still the best shot of getting a European car for a decent price, so for me it's either Sixt or Silvercar (which, if I'm being honest, I should have picked this time - I had a reservation but Sixt came in a little lower and I was hoping for the awesome 228i from my last trip).

Location: GDN airport
Status: Diamond
Booked: MDMR (Opel Adam)
Received: CDMR/ILMR (Mazda 3 Skyactive, manual 6-speed, petrol, no nav)
Duration: 4 days (actually three + 3 hours)
Driven: 300 km
Comments: Not a great experience at the counter - was told that only Opel Adam was available. My shock was huge and I emphasized to the agent that Diamond has guaranteed upgrade. He said, well you can wait until 5:00 pm (about 4 hours later) for a Mazda 3. No way! He types away and says "oh, I can offer you the Mazda 3 now". Couldn't believe a word he said afterwards. That being said, the lot was completely empty - there was literally the Opel Adam and the Mazda 3. Therefore had you rented EDMR for instance you would have received the Mazda 3.
Regarding why I put CDMR/ILMR in the "received" field - the agent said it's in the same category as V40... I couldn't believe it (and didn't remember it that way either) so I looked online and showed him. He showed me the station fleet list which had the Mazda 3 in both CDMR and ILMR categories (I have a picture). Not sure how different a Mazda 3 can get that it could be reasonably placed into both categories (or ranked together with a Volvo V40 and BMW 1).

Pick-up: SeaTac
Same drop-off
Status: Platinum
Booked: CCAR (Mini)
Received: an offer to upgrade to a Chevy Trax; app offered Buick LaCrosse or Toyota Camry. Paid an extra $20/day to get a Scion tC. Tons of fun, just not sure it was worth the extra $ since I didn't really drive it much (the $8/day toll charge wasn't worth it, either). Still, better than putter around in a retirementmobile.

Location: WAW airport
Status: Diamond
Booked: MTMR (Opel Adam Rocks)
Received: ILMR (Volvo V40 petrol manual, 26000 km)
Duration: 3 days
Driven: under 100 km
Comments: Was a great rental. Booked an hour before arrival (confirmed in 5 minutes though request only), paid a pittance, and had a great car to drive around in WAW. Didn't have to struggle or plead at the station. I like my V40s in Poland!

LHR
Platinum
XFAR - Mercedes ML350 or similar
XFAR - Mercedes ML350 AMG line*

*Technically no upgrade, but I wasn't expecting one. The only bigger vehicle would have been the GL350. Nice drive. ML350 was fully loaded. Booked SIXT as this rental was one-third the price of what Hertz wanted. Overall, very happy: rental process took a bit longer than with Hertz (no vehicle pick-up at T2, bus to and from rental facility and a bit of paperwork and no drop-off back to terminal. Added time total on both ends about 45 mins) ... but for big savings!

The LHR location is great. I was late for a flight one time and returned my SLK (upgraded from an Eco, hehe) to T5 erroneously (I messed up the return). On arrival, I ask the guy, "how far to Heathrow?" and he goes "you're right underneath it". I tell him "Sorry. I mean T2. I need to be there in 11 minutes else flight will be closed." So he just drove me to T2. Wonderful service.

Location: Nuremburg Center
Drop-off: Essen Hbf
Status: Diamond
Booked: CWMR (Mondeo Turnier, Golf Variant etc.)
Received: FWMR (Audi A4 Avant TDI manual + NAVI, 17000 km)
Duration: 4 days
Driven: 1400 km
Comments: Location very convenient (no premium location fee, good option if arriving at train station). Loved this Audi (navy blue colour). Was really perfect for the long drives and autobahn. Went in the previous day to tell them that I would prefer an Audi or BMW, and they said they'd see what they could do. They had a Jag F and a BMW M4 on the lot, but these aren't available for one-ways.

FRA
Platinum
Booked FDMR
Drove PDMR

Location: WAW
Status: Diamond
Booked: MTMR (Opel Adam Rocks)
Received: ILMR (Mercedes A class Automatic, NAVI, AMG trim inside)
Duration: 1 days
Driven: 50 km
Comments: Very nice car for this short rental - cost 15 EUR, didn't have to top up gas as the tank was full when I returned. Great deal. :) Merc A is very small when compared to other cars in the same class, a fact I was reminded of when I opened the trunk and could barely fit my standard suitcase in it. Just something to keep in mind. The AMG trim was very nice - bucket seats, nice suede and leather finishes.

Location: DFW
Status: Platinum
Booked: PCAR
Received: XCAR (Cadillac XTS)
Duration: 2 days
Seems to be standard at DTW. Very friendly front-desk

Location: FLL Sunrise / Airport
Status: Platinum
Booked: XCAR
Received: Usually XFAR (MB ML350 / BMW X5)
Duration: 10-30 days

I've been renting with Sixt in FLL for 4 months now due to working here and getting my credit score up for my own car (company pays). I usually book an XCAR, which goes for around $40 per day on these longer rentals and receive either a BMW X5 or a MB ML350. The X5 is much nicer and better equipped inside :)

I was dropping off my previous rental (BMW 5 series, no upgrade unfortunately) and picking up my next XCAR last week and they only had to offer an A6 (been "upgraded" to one of those previously, did not really like it). I then mentioned this to the customer service agent and then they asked which car I would like... I had looked around the lot when driving in and saw an S 550 so I pointed to the car and said "That one please".

Some commotion ensues when they check if it is free or not and then the agent says "Of course Sir, I will have that ready for you right away". I honestly did not think that they would give me that car, but apparently asking never hurts :)

EDIT: Still same XCAR price of ~$41/day.

Location: HAM
Status: Diamond
Booked: CPMR (BMW 1 or similar)
Offered: ILMR (BMW 2 series Active Tourer)
Paid upgrade: PDAR (BMW 5 series sedan)
Duration: 5 days
Driven: 1000 km
Comments: Very poor rental experience at counter. Offered only 2 series - it's an ugly car, and frankly I would prefer the 1 series I had rented. At the least I expected a BMW 3 or A4 of whatever flavour. Finally I negotiated to 15 EUR a day for a BMW 5. It took 45 minutes to get there, however. Got an Austrian-plate car, without the Umwelt zone sticker, so I complained when dropping off and received 20% discount on the base price. Car was quite nice, white leather finish, not too many km on it, but honestly it wasn't the rental experience I expected.