Sixt didn't tell us until we came to pickup that the SUV we'd booked was not available. 1pm: The agent wanted to downsell us for a much smaller subcompact SUV for the same price - and I was like... what? How does that make sense. After much back and forth, they gave me a loaner vehicle (AC was broken on the loaner - so they swapped it for another one - good thing I checked), and then I had to come back 6.5-7 hours later to check on available vehicles after 7:30pm. Basically - we lost 1/2 a day on our trip and everything was thrown off schedule. Too bad Sixt doesn't have a vehicle guarantee - they don't even notify you in advance (which is easy to do).
Once I got the vehicle I originally was paying for - everything else was good. Vehicle was in good condition, and loved driving it around. Return was easy as well.
TIP - they didn't do a pre-walkthrough of existing damages - make sure to ask them to send you that up front so you have your own copy. (They don't send you the existing damages until you return the vehicle - which is a bit strange).
Would book with Sixt again - competitive prices. But they aren't very reliable - so know that if you have a tight schedule.