It might be a good idea to avoid the RER
I am not sure it is a good idea to ***advise*** foreign, particularly English-speaking, travellers to use the RER. The situation is getting worse every night and those who do not speak fluent, unaccented French, do not dress like the French, carry laptops or suitcases, and speak with an American accent may not be totally safe. I am not saying it is dangerous--just unwise.
The Roissy Bus line is very fast, cheap, and convenient. It runs on the A1 motorway which should be safe from hooligans because of the heavy traffic.