Has anyone used Sixt car rental before? I've never heard of them before - but it looks like I can rent a Mustang convertible from them for the same price as a Toyota Corolla from the normal places!
Where is it located? Most of them aren't at the airport -- you either need to get a shuttle or a cab to get to the location. Depending on how much you value your time, the upgrade may not be worth it in the end.
When we were in Germany in 2009 we used Sixt and loved it! Our car was great and we were upgraded to one with a GPS at no cost. It was located at the Frankfurt airport and seemed to be quite common in each of the cities in Germany we went to (Bamberg, Munich, Koln and Frankfurt am Main).
We used Sixt in Switzerland for the majority of our rentals because it always ended up cheaper. They were always good to us and we got a great upgrade once. It was at the airport in Zurich and very convenient to our apartment. Don't know about other locations.
We used them out if orlando. Just look in to your route and if there are tolls. You can pay like $7 a day for a pass from sixt. Some toll roads in FL no longer take cash. It is all automated.