If this is the only way you can get to Key West for a one day visit, then it's a good plan. We grabbed our Ft Lauderdale shuttle at 5:45 am. I recommend bringing your own snacks and coffee. It was kind of cool driving through South Beach and Miami so early to see what it looks like when it's quiet. We stopped for 20 minutes in Key Largo for a bathroom break and to grab a quick snack at Dunk'n Doughnuts or the McDonalds it's not enough time, but it's understood there is a timeline. As other readers have said, there is a FREE hop on hop off bus running all day at the point where you are dropped off. The drivers were very good and upon our arrival back to Miami, they made sure we were the first ones dropped off to get our shuttle back to Ft. Lauderdale.