I've been living in Miami for nearly 2 years and decided to do the hop on hop off bus tour one Sunday afternoon to learn more about this awesome place. I thought it was great and wish I had have done it sooner. I started with the beach loop, had a quick break at Bayfront then jumped on the city loop. As I've already been to all the places covered on the tours I stayed on the bus, but you are free to get off and on at any stop. Each tour takes around 90 minutes. I will definitely recommend this to friends and family from out of town when they come to visit, and also friends who live locally. If you do want to get off at any of the stops I'd recommend doing these tours over at least a day, or possibly 2, as there is plenty to see. The guides were very good, especially the one I had for the city loop. Unfortunately I can't remember his name but he has lived in Miami for 50 years and was very knowledgeable.
My only two gripes are that the buses only run every 30 minutes which could be painful if you just miss the last bus, and also that you cannot purchase tickets for each tour separately (ie. if you only wanted to ride one of the buses you still have to pay for the two). But at least you get 24hrs to use the ticket. There are also some low hanging branches along the route but it is what it is and the guides were very good at warning everyone. It would only be a problem if you weren't paying attention.