We bought a Turbo Pass for our visit to Rome. This noted that it included the 'hop on / hop off bus' ticket for 48 hours. We saw large red buses labeled that way, so on our 2nd full day when we were ready to return to our hotel, we found the ticket office near a stop and went in to show them our documents. They informed us that they no longer worked with Turbo Pass (this changed annually) and that another similar bus line, IOBUS, was the one we needed. They politely directed us outside to a nearby stop. The bottom level was fully enclosed with windows; the upper deck had a roof, but did not have windows. It was fine if the weather was decent / warm.
The bus (with 3 colors, to distinguish it from the other similar companies) pulled up to the stop. We showed the conductor our papers and got a 48-hour ticket. We were also issued some cheap wired earbuds with which to plug into the in-seat audio ports. You could select a language and (theoretically) the volume. The volume controls were marginal in my case. I listened to the audio for most of our trip (around 1/3 of the loop around the city). It was okay, with some decent descriptions of the historical relevance of some sites.
We got a map also, and were thus able to find a relevant stop the next day in order to re-board the bus and get to a point reasonably close to the Pantheon. The Rome subway lines are basically a large X, and do not access a major section of the heart of the city. I can somewhat sympathize with this; how hard would it be to dig a subway tunnel here without unearthing so much historical debris that construction would get shut down? If you are adventurous, you might be able to navigate city bus routes. I assume those penetrate the areas not served well by the subway. However, the IOBUS was a good option for us during the 2 days we utilized it. If you had to pay the full price without it being included in a larger package, that might be a different proposition.
The route of this and similar buses does get you close to almost all the major sites in Rome. You may have a 5 to 10 minute walk at worst, but you can find a stop nearby. The subway only gets you this close to a few places, but is further from most others. Depending on your budget, the IOBUS may be a fair alternative to taxis or Uber. Do your research on pricing before you go.