Just back from Rome, we booked on the citysightseeing bus tour company. About €25 for 2 day ticket. I'd used them before in several other cities so looked out for them. What a terrible service, most of the earphone plug in ponts weren't working, we missed half the commentary jumping from seat to seat trying to get working head phone points. If you were lucky only one worked, and your companion had to move to a different seat. The commentary was ridiculously out of synch with the location.
Worst of all they seem to have the least number of busses of all companies. Nearly a mini riot at the vatican stop, people were waiting for ages in the heat, and multiple busses of all other companies seemed to arrive in the meantime. When a bus finally came there were so many waiting the organisation of the queue was a farce, latecomers getting on first, lots of irate people, an unpleasant experience. They have just been having a bad day, and I had gods experiences in other cities, but this was pretty poor. Choose a different one, thete's lots and lots of choice.