I rarely take any guided tours; I prefer to move at my own speed (or that of my family), consulting from a variety of in-depth guidebooks (such as the "blue guide.")
But I have taken Context Travel tours twice, once in Rome and one in Venice (different trips). And my husband's birthday gift to me this year was a tour of my choice that we'll be taking in a couple of weeks when we're on vacation in Rome.
I prefer being in a small group; it's easier to get around and to hear the guide. That's probably a matter of personal preference..
The guides are very knowledgeable, usually PhDs or PhD students, and have a lot to say and tell. On the Rome tour, the guide was good at dealing with me, talkative and a history geek, as well as the other people on our tour who were in Rome for the first time, and who were more interested in learning the basics.
On both of those tours, we chose topics that were more specific. I don't remember the tour in Venice, other than we went into several lesser-known churches. We took the Palatine (and Colosseum) tour in Rome; because that area is more ruinous, I felt it would be helpful to have a guide describe the area. And it was.
Yes, the tours are expensive. But if you want a very good small-group tour, led by knowledgeable guides, then consider Context Travel.