Without a doubt the drive through the Keys is absolutely GORGEOUS!!! Nothing but bridges (including the famous 7-mile bridge) and beautiful water, BUT the two-lane driving is NO joke!!!
I flew into to Miami and rented a car to drive to Key West! The drive is about 3+ hours long. You will go through the MANY different keys, but there are plenty of places along the way for you to stop for a rest.
I don't think personally that I could do the drive again, since there are services that will take you from Ft. Lauderdale, Miami, Ft. Meyers Beach, & Marco Island to Key West. Check out: Keys Shuttle and Key West Express!!
But if you are up for the drive, pack a lot of PATIENCE and have you camera ready!