My friend and I did Lake Worth to Key West only, so I can't give any information on Northern Fl. But for Southern Fl. below Florida City is rough. There are only two roads IIRC that lead from Key Largo to Florida City, and both are two lane highways. There is a lot of construction going on and we had to walk a few miles along that stretch. Above that though, you're pretty solid. From Florida City to Miami there is a Charter bus road alongside US1 that is empty but for a bus every 30 minutes or so. After Miami it's A1A, which is pretty cyclist friendly. I've ridden A1A pretty far north on other rides, and from what I've read, it goes most of the way up the coast.
Good luck, I hope some of this info helps.