Old decrepit system but it does get you from point A to point B. Would recommend before you use the system that you do lots of research on-line to understand the ins and outs of the system. Would recommend visitors purchase Paris visite pass tickets (each day expires at midnight) over the Navigo travel passes with unlimited rides on the metro, RER, buses. Buy your tickets only from the Metro official booths. Make sure you fill in your name and dates for use and validate your ticket each time you enter train station/bus etc. If you don't do this, be expected to be fined if you are checked. The tickets also get easily demagnetized so don't store them with your phone etc. Know where you need to go before you get in the station. Watch out for pickpocketers!