It´s true that the Medellin Metro system is great. Very clean and lots of guards riding the trains and on the platforms. As I´m a fan of subways (the ones underground), it´s disappointing that this system is entirely overground, actually elevated like Chicago. This nice thing is you can connect for free to a cable car and ride 3 stops up a hill and back as part of your 1150 ($0.50) fare. A nice first day thing to do as a tourist....
Subway runs above ground actually - mostly along the river, and in the center on viaducts, so this is more of a light rail system.
Trains are very clean, smooth and nice. Stations are very well guarded. The ride is cheap - only a few hundred pesos.
The international airport is located well above the city, and the road there is long and windy. You will be actually driving through the clouds on your way to or from the city. Budget at least an hour for the trip!
Taxis here are plentiful and cheap. Seriously, you can't imagine what 'lots of taxis' looks like until you come to Colombia! Mexico City or New York City barely have any in comparison.
My tip is to pick a Hyundai Atos taxi, if any are available. They look ugly and funny, but they ride really really comfortably.
Medellin is the only one of Colombian cities to have a subway system. The subway is relatively cheap, clean, fast, safe and quite well structured.
By air of course! From Miami there are direct flights on Avianca or Aces; also connecting flights. Within colombia it is advisable to travel by air rather than by road.