I can't provide feedback about renting from Sixt, but here's what I know about the shuttle.
The Sixt shuttle picks up outside the Rental Car Center on the departures level. On the map below, the stop is just to the left of the connector across from Terminal 1. If you're arriving in Terminal 1 (on Southwest, Alaska, Allegiant, Frontier, Silver, United, Virgin America, or WestJet), you'll be able to access the Rental Car Center directly.
If you're arriving in Terminal 2, 3, or 4, you will need to take an FLL internal shuttle bus to reach the Rental Car Center.
Once at the Rental Car Center, you'll go outside and wait for the Sixt shuttle. The office/vehicle location is roughly 2 miles north of the airport. It's really about as close as you can get without being on airport property...probably 5-8 minutes. Still, the double shuttle (if not using Terminal 1) will certainly add a decent chunk of time to the process. Plus, there's no guarantee that the Sixt shuttle will be ready and waiting for you.
