We took the RER into the metro about one year ago, we arrived from SEA around 630 pm and we were fine. The train was quiet and then we rode and transferred our way to the Hilton ADT. On our way out of France, we stayed at the Hilton CDG and no upgrade, but did get lounge access. At the ADT we did get the upgrade.
I think the train is fine, during peak times there may be more people, but I didn't think it was that bad myself. You can always take one of the buses right outside the terminal and I think it's around 30 Euro for a one way trip.