We have used them several times in other cities and they are good.

H O W E V E R, we booked them on SXM only once and we couldn't find their shuttle at the airport and eventually took a cab and skipped the rental got the whole trip.

I would get clear directions from them where their shuttle will be, how often and if someone in the airport is willing to call them. No one at all those desks seemed to know anything about SIXT except go out to the shuttle area for ,