Generally very good. Great, helpful staff and the (very) off-airport location not too much of a problem. I ordered a BMW 430, which came out at £64 per day, compared to a similar car rented from Hertz in Zurich at £80 per day. But the Zurich car was much better specified. Things I would have expected as standard (e.g. heated seats and heated door mirrors) were missed, and other features (e.g. uprated stereo, head-up display) could have been expected.
A particular problem was the GPS. I ordered an extra GPS, not knowing if the American spec included it. However, no GPS set was supplied, although it appeared on my ticket as a charge. Even worse, the car came with GPS as standard - which didn't work. It could find a previous location, but not a new one, and so was presumably missing a SIM card. Having thought I had two safe options to navigate, I found that I had none, and had to rely on my passenger's Google Maps via a cellphone. I explained this on returning the car, and it appeared that the GPS unit was not then charged. It was poor that I had to resort to Google Maps, though.
Other issues include the fuel level was not full on delivery, being 7/8 at best, and damage to the windscreen had not been noticed.
These things can, and should, improve, and I would happily use Sixt again if you can maintain your price advantage over others.