I can't decide if I should take the Hop-on Hop-Off Trambus when I first arrive on a Monday afternoon or go to the Pantheon and surrounding churches instead. All other days of the week are pretty packed.
It is somewhat personal preference but in new cities we like the hop on/off buses to give a good overview of all the main tourist sites. Generally the tickets are good for 24 hours so you use the buses the next day as part of your transportation.
I read that they limited the stops on the HoHo buses this year stopping not too close to many sites. You should look at a new map and and make sure it stops where you would like.
Just so you're aware, as mentioned, there are limitations on the number of places the Hop on - Hop Off Buses in Rome can stop. Due to the roads and locations, the HOHO buses do not go by the Pantheon, the Spanish Steps, or the Trevi Fountain. So you won't be cruising by taking pictures.