I have taken the RER to CDG a number of times without any problems. I have heard from a couple of travelers (not those inclined to exaggerate) that this is not really a safe idea? I would be curious if other folks had noticed any changes?
I have taken it alone.. and with my kids.. as late as last summer.. its safe.. I think your friends are not really that experienced... Keep an eye on your suitcase and keep your wallet safe.. but other then that its perfectly safe.. look around.. its filled with women and children ( like me) and we aren't scared.. lol
Chances are you won't be the only one with luggage on the RER heading to CDG. Women and children (tourists among them) will be on that RER too. Yes, it can be crowded. Basically , stay alert. Those travelers advising this option of going to CDG on the RER is not a good one based their opinion on fear. Nothing to be fearful of. I've taken this route a few times too, mostly solo, and sometimes it is less crowded.
The last time that I took it in 2013, the 7:00 a.m. RER departing Chatelet les Halles was delayed and was quite crowded, but I did get one of the last seats and had no other issues.
Well, I do know an experienced traveller who had a computer crammed with scientific research lifted on the RER from CdG. Proves nothing. Thousands take that train every day and almost all without incident. But it costs nothing to be extra alert and keep all bags close under hand.
My husband and I took the RER from St Michel to CDG in Oct. No problem. Very easy.
December 2014. CDG to Luxembourg and back, no problems. I've taken RER to/from the airport numerous times both by myself and with company.
The wife and I rode the RER this past November without any problems, and didn't see anything out of the ordinary.
We have taken it many times. It is fine. ALL public transport in Paris is infested with organized pickpockets. They also operate in museums and other tourist spots. The RER to the airport is no different. You cannot carry valuables in pockets or backpacks. You must keep your hands on your hand luggage like the computer bag. Do that and you won't have problems. Petty theft is common in Paris mostly by stealth, sometimes by misdirection (e.g. leaving your bag on the seat when someone falls and you spring up to help while the accomplice grabs your computer bag). There is virtually no chance of violent crime which is so common in the US.