Last week I just rented at last minute from SIXT as my 5 day cost with Avis was going to be $260 and with SIXT even at the last minute I got $134 a/i for same period.
They also told me there rates are the same all year, no black out periods. Previously I have rented at Xmas with Alamo and Avis at exorbinant rates.
I love SIXT, and they have a location at FLL in the car rental centre as well.
Their desk is small at MIA (only 2 or 3 checkin desks available) and I suspect at FLL too, but I did not have a long wait. They were very accomodating and very nice to me. In comparison the line up at Alamo was huge.
Bye Bye Avis!