We have used the familiar red hop on hop off buses in many cities and have thought them to reasonably good. Not in Rome. The route is quite small, the stops are not well placed and identified and they appear to sell too many tickets sold for the number of buses. At the Coliseum we had to wait for 4 or 5 buses in order to get a ride. In Rome, the tourist sites are not so spread out that you can not walk or combine walking with public transport.
If you want to go hop on hop off maybe try the green bus.