Excellent experience all around! The service was impeccable and extremely hospitable. The car was clean and well-maintained. This was my second time renting from them and both times I was impressed with the people who worked there.
One thing I will state is to make sure that you take pictures of your car upon getting it. I like that they send you a link where you can upload pictures of any previous damage.
The first time I went, I didn't receive the link in enough time and couldn't take pictures to document the damage, although I did it on my phone. I'm so glad that I did because I was contacted about the damages and they initially said they were going to charge me but when I sent them the pictures and video that I took before hand, they notated it and told me they would take care of it.
But I recommend doing that with all rental cars, not just Sixt because you always want to cover yourself.