During rush hour (6pm) on any day of the week, you may have to stand a lot and be packed in with people.
You can purchase a 5 day pass at Expedia for about $45 which pays for your bus, RER, and Metro transportation. For all practical purposes, you will spend most of your subway traveling on the metro lines 1-15.
If you purchase a subway pass from Expedia, you will most likely have to go to Gare du Nord Station at the Tourist Information booth to pick up your pass and useful map. I walked from my hotel on Rue de Citeaux all the way to Gare du Nord which took me about an hour and a half.

