I have a Sunpass. You actually pay a little less per toll than if you pay cash.
There are no extra fees for Sunpass. I have set mine up to auto-fill whenever the balance gets below $10.00. There is an extra $2.50 charge per transaction for the Toll-By-Plate system.
I believe you cannot transfer the Sunpass Mini which sticks to your windshield unless you sell your car. They do offer a Sunpass portable. Since you can maintain whatever balance you want, there would be no expense or reason to not have one for each vehicle.
Here is a link to a video which does a good job of explaining the program.
All-Electronic Open Road Tolling