We booked through Viator. Everything went very well. The guides were at the meeting point on time, had a sign, and gave us headsets during the tour. Our guide Sara was nice, enthusiastic, and gave us a good tour of all the sites listed in the booking info.
We saw the inside of the Colosseum and learned more about the details and architecture.
Not much time to take photos so do so quickly as you're strolling through the locations. Or you can do what we did and took photos outside the colosseum before the tour started. We were able to skip most lines and only waited briefly on the few lines we were in. Wear comfortable shoes, bring a hat, umbrella, and water/snacks. There's also stair climbing. The group size was just right. It's a long walking tour but worth it to learn more about the area.