I've lived in Florida now for about 15 years (Orlando). Sunpass was the first thing I got when I moved down here. It's really a no-brainer. Either sit in the cash lanes on SR 417 or drive by and wave. I am still astonished at the number of cars with Florida plates sitting in the cash lanes.
Lots of places to get them including Publix Supermarkets (make sure to pick up a Key Lime pie while you're in the store - Best $8 you will ever spend).
My advice is just do it. There is no reason not to. Avoid the hassles. The money doesn't expire. And I believe you still get a cheaper rate on at least some toll roads if you use a Sunpass instead of cash.