The new large cycling strips that appeared in the last 5-6 years are so good. At commute time there are frequently jammed with /cyclists/, but let's face it it's miles better than being stuck in a car. I shudder to think about the alternative where each cyclist was instead alone in a small car, this wouldn't even fit on the roads.
You haven't been in a bicycle-jam until you've been before an open bridge just before the university colleges start in the Netherlands. Hundreds of cyclists trying to squeeze through a tiny bottleneck. Still costs less time than by going in a car.
I do wonder how many cyclists in Paris are really replacing cars versus replacing metro usage. Obviously, it's still good for people to cycle as well since the metro can be insanely crowded at times, but living in Paris, my impression is that the people who cycle are the kinds who would have been unlikely to own a car in any case.
On a nice day it's fantastic to be out, but Paris can be cold and rainy. They really need to have a plan for those days, too.
Paris Metro is pretty nice, and reaches most of the car free area. But I'm not sure if it can handle all of the cyclists if they're all trying to avoid a déluge.
I would love to be on what amounts to a group ride to and from work safely. That has to do wonders for all kinds of things both physical and mental. If it were safe I would do it year round.