I used Metrobus eight times, and have only positive things to say.
It links the parts of Miami that the various train systems don't. It only costs $2.25 and tickets are valid for three hours (great if you need to change buses).
On my first journey from the airport to my accommodation I asked two bus drivers for help about where to get off, and they were superb. After that I got the hang of it!
Two problems. Visitors should go for Easyticket which can be loaded with as much cash as you want. This is different from Easycard which is more suitable for locals. At the airport the two tickets confusingly are on sale from different machines.
Buses are not fantastically frequent, so look up the timetables on-line.