I was in Rome a month ago and booked four tours with Dark Rome Tours (each about 4 hours, 2 per day), all went perfectly well, the tour guides were well-versed in the subject and enthusiastic. Bad thing for me was that The Vatican closed the Sistine Chapel without warning (tour companies have no control over it) the day before my tour of the Vatican, and when I landed in Rome, I had a text and email telling me about it and offering refunds and options.