My wife and I spent three days in Rome in the Fall of 2010, did get the Roma Pass, and did not regret it. We used it for the Colosseum and adjoining Palatine Hill, as well as the Borghese. No line at the Colosseum was great. I'm not sure how a "no line" would work at the Borghese with militantly rigid scheduled entry times.
Even though you can walk to sites, we made frequent use of the metro to cut down on some of the walking. We'd pick an out point, take the metro to it, see the site or sites (i.e. metro Colosseum, Palatine, Forum, past the wedding cake to the Pantheon) and walk back.
We still logged like 8-10 miles a day on foot while utilizing the metro.
We found it easy to use and as a tourist, I'd feel much safer in the Rome metro than in NYC. Just be aware.
All in all, I found the pass worth it because we did visit multiple attractions as well as make full utilization of the metro. I am not sure if we broke even Euro for Euro expense vs. savings, but the convenience factor made it feel like a deal. I recall even quipping "Roma Pass... don't leave Doma without it."