We just returned from the Keys. Last year we flew into Ft. Lauderdale and rented a car. I purchased a sunpass mini for $4.99 and loaded $10.00, allowing $5.00 for each way. Then I just put the sticker in the rental car windshield and removed it once we got to the airport to return the car. This year we drove from New Jersey so I ordered a new sunpass mini under the same account I created and put the same amount on it. It was super easy and much more economical than paying what the rental companies charge. They charge a daily fee on top of the tolls, even if you are not using a toll every day. I also would not take my chances doing nothing and having a charge later on my credit card. The Sunpass website explains everything and it is super easy.
Also, I felt the Florida Turnpike is the best way to get down to the Keys from Ft. Lauderdale.
Enjoy!