to modify alex's comment that using the rer can often be much more practical than the metro, this is absolutely true--sometimes. first off, if you stay within the city proper (i.e. zones 1 & 2), you can use a regular metro ticket to ride the rer. BUT you MUST SAVE THE TICKET IN ORDER TO GET OUT. the rer requires that you have your ticket to get in and then you have to have that same ticket (or purchase another but why would you want that) to put it through a turnstile to leave as well.

the rer also goes to handy destinations like versailles, disneyland and CDG (the last is about 8 euros each way).

however, one thing to keep in mind is that it is primarily a commuter type rail and so if you're boarding at rush hour times it can be pretty excruciating to get on--lots of sweaty bodies and bad tempers. also, it runs on a schedule so, unlike the metro where if you miss one, another is sure to come along in 2 min, here, you can be waiting around for up to 15 min before the next one arrives.

wasn't my favorite form of transport, but it has its purposes.

all best, jeannine in sf--where in august you cannot step out without tripping over a french tourist.