My experience was the same as Texbiker's. There was either path or generous shoulder 99% of the way. Only a couple of miles that were sketchy for me.
I actually rode one day earlier than he did last year (caught the rain between Marathon and Key West and it was a welcome respite from the heat).
There was a slight headwind that weekend, but from what I've read elsewhere the wind usually pushes you south in the spring.
The lizards on the path were fun, although I can imagine that they'd spook some people.
I rode from Miami most of the way to Key Largo, but got caught in the dark on the bridge going over Card Sound. When the sidewalk ended and cars were whizzing by in the pitch black, I called it a night.
I'm looking for a better route from Miami to Key Largo for next time. The path by the busway was wide and mostly smooth, but lots of glass (two flats) and not very picturesque. If anyone has a better route, please share.