Two attractions in Rome that need definite pre booking are
- Colosseum tours
- Vatican museum
Roma Pass is not valid for both the above. Hence, you need to pre book the above attractions even if you are going to buy Roma Pass. Also, do not buy Roma Pass for its skip the line privileges. Buy Roma Pass only if makes economical sense.
Colosseum tours and Roma Pass
Roma Pass is an entry pass which means you can get entry into Colosseum. However, the pass does not include any tours. You have to make seperate reservation for the underground tour when using the romapass. Call the operator or book online. With Roma Pass, you need to pay only for the underground, third ring and belvedere tours and not for the entry.
Personally, do not see a big advantage in Roma Pass.