It is a small nuisance, but the portable sunpass device works very well. We have had ours for 2-3 years now, and have used it in a number of rentals and our own cars when visiting FL. It easily attaches to/detaches from the windshield. You have to update your sunpass account only when you are using a rental, as mentioned before, but that is easy also. And that update includes an end date for the rental, so only one access is needed.
We have EZpass and are looking forward to the changeover, but we like how the sunpass is portable.