A Tourist in Rome - Roma Pass
The Roma Pass can be purchased for €34 (all prices in this section are as of Sept 2013). It gives you a 3-day transit pass (worth €16.50), gets you into your first two sites for free (if you select the Colosseum/RomanForum/Palatine Hill and the Capitoline Museum, that's worth €15.50 and €13), and gets you about €2 or €3 off other sites for 3 days from the first use of the pass. The pass is usable at the sites listed on the Roma Pass web site, but they do not include the Vatican Museum. An added benefit of the Roma Pass is that you don't have to wait in the long ticket lines and entrance lines at the Colosseum -- this saves you about an hour of your time. So you pay €34 for, perhaps €50 of benefit. Sounds like a no-brainer, right?
Not so fast. If you'd bought a 7-day transit pass, the cost would be €24, so the 3 days of transit pass you get are really only worth €10.50, not €16.50, so the total benefit is now €44. And what if you've been to the Colosseum, and plan to use the Roma Pass at the Borghese Gallery (€11) and the Museum of Imperial Forums (€11). Now your value is €37.50, just a little more than the €34 cost. So you need to be careful and consider what you plan to use the pass for, how much you can see in the 3 days the pass is valid, how much you'll use public transit during those 3 days (remember a single ride costs only €1.50), etc.
However, if you're going to see the Colosseum and the Capitoline Museum (two of the more expensive attractions, located very near each other), and you're not going to be in Rome for long enough to take advantage of a 7-day transit pass, and you plan to use public transportation a lot during those 3 days, then I think the Roma Pass is a great deal.