I've been and have never been back and unlikely to go back. There are many greater cities and areas of the US to go to then Miami. If you list the sort of things you are looking to do then I am sure you will get some great recommendations.

If Miami or Florida appeals, head to Fort Lauderdale, spend some time in Miami if you must and then head to the Florida Keys. Very laid back and chilled out vibe and Key West is amazing.

Depends what sort of a holiday you like. If you like sightseeing then head for California. New York is great for people who like cities. If you like a beach holiday, as the other poster says Fort lauderdale is nice. Naples on the gulf is good too. I would avoid key West. Used to love it but it has changed and there are now a lot of tourist tat shops. Personally I do like Miami. South beach is cool. Lots of shopping malls, museums and art galleries. Calle ocho is great for eating and drinking.