Had the 'pleasure' to rent from Sixt in LIS last week. Totally regret it, hence the report here:
I and two colleagues were on a business trip to a supplier about 200km south of LIS and I had rented a CDMR with navigation for two days from Sixt, one of our corporate contractors (the others are Avis and Hertz). I basically went to the minisite we have at Sixt and just booked blindly. Just out of habit, I printed out the booking confirmation to have something in hand in case my phone ran out of battery etc. The ics file that I stored in my calendar stated the location as 'Sixt Lissabon Flughafen , Av. Severiano Falcão Lote 2, 2685-378 Prior Velho, PT'. Two days before arrival I got the usual 'Ihre Mietwagenreservierung 9823459959 wird vorbereitet' which I noted, but did not really read.
Upon arrival, I headed to the rental car counters and noticed that Sixt had no counter at the airport. Ah, so they seem to be off-site. Fair enough, out comes the printed reservation with details on where to find the shuttle. Once we found the spot, a lady in a sixt t-shirt was waiting there clipboard in hand which also had my reservation details. The shuttle pulled in to the kerb in front of the building and we hopped in. After quite a while (not the 5 minutes in the confirmation, rather 10+) we arrive at the sixt location which had a line out the door. A quick look inside revealed 2 counters and about 9-10 parties ahead of me. Quick math (10*5min /2) revealed me that this would be a wait or maybe 20-25min, but not really bad. So the colleagues were waiting outside having a smoke and I stood in line. Thought about waving my platinum card to jump the queue or demand a separate treatment, but DYKWIA is not really my thing in such cases. The station is branded Sixt, but actually operated by JAP Rent-a-Car.
After about waiting for 20 min I noticed the line was not moving at all. The agents were taking 15-20 min to process each customer, painstakingly explaining all details, selling insurance, the via verde toll system etc etc. First I was curious why they would do the hard sell with such a long line, but then was told (by a FTer via whatsapp who had worked in the industry) that they were stalling because they were short of cars

.
In the end it took me 90 mins to get to the front of the line. Once it was my turn, I noticed that their system, although Sixt on the surface didn't get any info on my reservation except for the barebone details. No sixt customer number, no FFP, no status, no nothing. They had to reenter pretty much everything, including passport details, credit card info etc etc. Of course the fact that I planned to collect 3x AV LM as part of their current promo got dropped which I noticed as I observed the agent entering my M&M number of my credit card without even asking me.
After standing in line for so long I new the exact answers to the questions they had in store, so replied quickly to Insurance, via verde, etc etc in a no, yes, no, yes, no, no staccato. I had timed my agent, and he actually managed to process me in 7min. Which was lightning fast compared to the previous customers. Anyway, still no time saved, as my car was not ready. He led me out to the checkout area and let me stand there with the papers only to vanish behind the scenes. Since I had the plate number already I walked around looking for my car, and sure enough it was in the shop getting a wash.
All in all it took us two hours from landing to leave the rental station, which in my petty view is far too long for an airport facility.
Upon return, you are not able to drop your car and keys and hop into the shuttle. It is mandatory to wait for an agent, who then inspects your car, prepares a final invoice and voids the 1000€ authorisation on your credit card. This took us 25 mins (because they were busy

) which made us miss the shuttle. The confirmation and rental contract asked to budget 20min for a return which is probably fiction. The season has not started and I don't even want to imagine the chaos they will have during the normal summer rush.