We rented from Sixt in Miami in March. We had the same skepticism and the one in Miami had some bad reviews from a while ago about them claiming the gas was not full and a few other things. However, it seemed that things had improved.
We will rent from them again, especially now that they are in Orlando. We rented about 4-5 weeks out a Camaro convertible for $90 total--for 5 days. We were next in line. The
Alamo "people" line was easily 30-45 minutes long and there was a line for the kiosks.
Budget and National were the same. Sometimes my time is worth money--especially on vacation when I've already had a flight delay.
It's a well-kept secret so far. As far as the hold, nothing appeared on our credit cards. No big deal.
If you find a deal you cannot pass up, they are a fine company--just German, so we are less familiar. They are plenty popular in Europe. We think they are just working in a more European manner and have found that these same policies do not work as well in the US.
Good luck.