I have used the Tri Rail from West Palm Beach to Ft. Lauderdale and it was a great experience. There is plenty of room for your luggage on the luggage racks or you can store smaller items by your feet when you sit down.
Make sure to be ready to get on/off when you get to your destination, the doors do close pretty quickly, but they will wait if you are in the process. It was very inexpensive too. I think for three of us it cost $30 total round trip. It may have been less than that. If you are going from the airport, it requires that you take a bus from the airport to the actual train station, at least from the Ft. Lauderdale airport. I'm not sure what happens in Miami. Everything is marked well.
We are cruising from Miami in March of 2013 and are intending to fly into Ft. Lauderdale and take Tri Rail to Miami. There is a website for it, I think it's www.trirail.com. If that's not it, Google it. It gives a lot of helpful information.