Has anyone taken one of their tours? Do you stay on the bus the entire tour and just see the sites as you drive by or do you get off the bus and actually visit the sites?

Is that supposed to be a serious ?

Yes, you do get off the bus. Obviously and depending on the length of the tour, however, you do not get detailed tours of many sites.
For example, you'll probably walk around the exterior of the Colosseum, but not go inside on the tour. Still, you will go inside some sites. We visited the interiors of Saints Peter and Paul in Chains and the church with Michelangelo's Moses (I forget its name).
We felt it gave us a good overview of the old city.

We did one of their tours out to Tivoli and it was fine - we had plenty of time there.

However, I would not recommend this type of tour - from anyone - in the center of rome. You will see much more by simply doing it yourself - and you can then stay as long or as little time as you like at each sight depending on your interest. Inmy expeience an in-depth guide book provides at least as much info as 95% of these "guides".

Can you book these tours while in rome

Yes, the AmEx office is right near the Spanish Steps. You can go and reserve in advance, or phone or have your hotel concierge phone for you.

BC