For me it's less about hating or not hating the RER. When measured against what modern fast urban rail service should look like I do indeed hate it, but still use it if it serves my purpose. But leaving home 30 minutes earlier and sitting in the back of a comfortable taxi where I can make phone calls and work on my laptop/iPad or read the newspapers or listen to the radio and have my luggage in the back vs. getting to the metro with my luggage, getting on to the overcrowded metro (this is rush hour), having to change Metro lines to get to Chatelet, then walking through the ghastly corridors of Chatelet Les Halles to get onto another crowded train and stand on my luggage, praying that the train won't stop somewhere along the way to CDG (which may happen given that it passes by the area where they do the COP 21)... no thanks. It's wasted time because all I do is be stuck on a train and it's uncomfortable. And I am not even sure it's faster. Hence, car it is for me.