Is the Roma Pass Worth It?
[Last updated with prices, etc. in September 2019...the 3-day pass *used* to be €30 and thus a better deal, but as of 2014 it costs more; prices for the Borghese went up around 2016 but so did the 3 day pass price]
Rome has the "Roma Pass", a 2- or 3-day transit and museum pass marketed to tourists as an easy and economical way to see the Eternal City...but is it worth it?
Let's break it down:
There are two Roma Pass options: 3 days (72 hours) for €38.50 and 2 days (48 hours) for €28. The first gets you into the first 2 museums/sights for free, and the second one gets you into 1 museum/sight for free. (Most major museums in Rome are included, except the Vatican Museums/Sistine Chapel.)
Most people will want to use the pass for the Colosseum/Roman Forum/Palatine Hill (€12 --> 16 combined ticket starting Nov 2019, although online bookings cost €2 more) and/or the Galleria Borghese (€12-15 ticket for non-EU citizen adults, similarly online booking costs €2 more). Also note that as of March 2019, Roma Pass holders must reserve online (same €2 fee as you would without the pass, or for free the same day at the ticket office...but if you show up the day of, they may not have availability) at the Colosseum/Roman Forum/Palatine Hill. Roma Pass also doesn't include S.U.P.E.R. sites (see this handy explanation).
Update: Starting June 2019, the Roma Pass was "temporarily suspended" at the Galleria Borghese --> this means that for most visitors, the Roma Pass is almost certainly not worth it.
Assuming the most expensive tickets, the 3-day pass would then be worth €16 + 15 = €31 which means that you're spending €7.50 for a 3-day transit pass. Since a "BIT" one-way transit ticket costs €1.50, you'd therefore have to ride transit at least 6 times in those 3 days to make it worth your while. Most people won't.
The 48 hour pass is an even worse deal. If you use it for an otherwise-€16 Colosseum ticket, then you are paying €12 for a 48-hour transit pass, which means you have to ride transit 9 times in 2 days to make it economical.
(Oh, and keep in mind that the Roma Pass's transit benefit does not cover more expensive journeys like going to Tivioli or the airport—it only works where a normal one-way ticket (BIT) would work. There used to be a Roma è Piu pass that would cover public transit in the entire region of Lazio, but it isn't sold anymore.)
Bottom line: This pass is almost always a bad monetary deal
1. Skipping the Colosseum Line
The Roma Pass does let you skip this very long line (for buying tickets). However, there are other ways of skipping the ticket-buying line such as buying tickets online (although that does incur a €2 extra fee), buying them down the street (at the Palatine Hill), or going with an organized tour group.
2. Lots of Museums
Since the Roma Pass gives you discounts at other museums (beyond the 1-2 free ones), if you want to pack in a lot of museums, it might be worth it. However, most tourists simply won't have the stamina for the number of museums it would take to make this a good deal (...and keep in mind that the most popular museum, the Vatican Museum, is not included).
Some people just want to buy 1 thing and be done with it—nothing more to worry about or lose. If you're one of these people, it might be “worth it” for you to buy a Roma Pass to avoid stress, even though you'll be spending more money than if you bought individual transit tickets, etc.. However: keep in mind that if you want to use the Roma pass to see the Borghese Gallery, you cannot reserve your spot online—you have to call (see the main Rome page for more details).
If you have children, the Roma Pass is usually a bad deal, and you definitely shouldn't buy one for children under 10. For example, children under 10 years of age travel for free on public transit, and most sites are also free for <10 with an accompanying adult. Many sites also provide discounted tickets for 10-18 year olds, while the Roma Pass costs the same for each person.
What about other passes?
There are now a bunch of similar cars such as the TurboPass, Omnia Vatican & Rome card, and Archaeologia Card. Amanda Ruggieri at RevealedRome has a great breakdown of these, and also concludes that for most people, they aren't worth it. RomeWise also has a pretty good breakdown of both the RomaPass and Omnia.
Note: a lot of other websites are spreading frankly false information about the Roma Pass, are out-of-date, and/or are making money off of referrals to sites that sell other cards. For example, this Budget Travel article incorrectly states that Roman Forum tickets are "$24" (??), and in any case compares the pass to buying a 3-day transit pass (not the one-way tickets most people would actually buy).