I would have to agree with others, it's a long drive to do straight off the plane, especially with a max speed of 55 (most at 45 or less). Last time we were in FL, two years ago, we actually did a day trip to Key West, that was a mistake. It really is easy to forget how far apart things are in the US.
This time round, we are flying LHR to MIA and driving about 90 minutes north before stopping in Juniper Beach overnight. Then driving up to Orlando for 2 weeks before heading back to Miami for four nights then down to Key West for four nights before heading back to Miami for the flight home (upgraded to PE, something to finish the holiday nicely

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If you've never driven on the Keys, it's definately worth it at least once, especially if you have a nice car to do it in. Make sure you are driving down in daylight though.
I've thrown a couple of pictures and a time lapse video of the sunset we took last time on Flikr if you want to take a look. Forgive the quality, they were all taken with an elderly camera and some taken by my wife out of the back of the car. It was a beautiful sunset though, this time round, I'm looking forward to watching it from a bar (looking through the suggestions in this thread) instead of from the side of the road.
5 days and counting!!!
http://www.flickr.com/photos/56768692@N ... 569255900/