We have taken the hop-on hop-off our first day in many many cities, and are almost always happy with it, despite the uncomfortable earbuds, faulty audio, etc.
This is the only place where we were truly disappointed. We realized that most of the interesting attractions are not accessible by bus, and that the sites are close enough that you can walk everywhere, but still thought we would learn something from the trip. We were wrong. The explanations were trivial and uninteresting, and many of the stops seemed to be inserted to fill up the trip.
In Florence there are 2 lines with 90% overlap between them (except for traveling in opposite directions). We visited in winter, and so Line A was once an hour, and line B only three times a day. Line A (blue line) has 17 stops, and you can transfer to Line B (red line) and Piazalla Michelangelo. However if you want to take a short stop somewhere you have to wait close to an hour for the next blue bus, and red bus stops not on the blue line are useful for stops.
The only truly additional feature of the red line is the trip to Fiesola. However, there is a 15 minutes stop there with no explanation, and it is not even timed with the local establishments - so you can't even buy a gelato or a coffee.
So, the lines save you climbing the stairs to Piazalla Michelangelo and can bring you to Santa Croce, but that's about all.
In short, skip it!