I suppose a logical corollary to any discussion of food and drink is a discussion of bathrooms. Public restrooms are indeed, as I had heard, much more scarce in Paris than in most big US cities. However, although I often had to wait in line for a public restroom (nothing new there; I'm a girl so I'm used to it), and sometimes even pay to use one (typically half a euro or less), the European restrooms I saw were much more private, and I never encountered a really dirty one. The doors and walls in the stalls typically go clear down to the floor, which was a pleasant surprise.
In an apparent response to the bathroom scarcity, public toilets appear in surprisingly roomy sanisettes at random intervals along Parisian streets. (The standard model evidently does not accommodate wheelchairs very well if at all, but for a public toilet sitting in the middle of the sidewalk, they are more spacious inside than I had expected.) A map of the neighborhood typically appears on the outside of the sanisette; as you approach one, you may be unsure whether the people clustered outside are waiting to get in or just consulting the map. These unisex bathrooms are cleaned automatically after every use. Many of them are free, and my understanding is that by 2014 all of the ones in the city of Paris will be free.
The first time I used one turned out to be one of those adventures that are more amusing after than during, but the second time was much nicer. For starters, the first time I used one, it took me a long time to find one so I was mildly stressed to start with. Then I was not sure the door was locked. After I got in and pressed the "close" button on the inside, I couldn't understand the voice that greeted me in French as soon as the door was closed, mostly because it startled me to be spoken to at all. I wasn't sure if it was telling me the door was locked or telling me to lock the door, and I couldn't see an obvious "lock" button. So it wasn't 100% clear to me that I had done all that was necessary to avoid being walked in on, and I was on a very busy street corner. (I learned later that as soon as you close the door, the "occupied" light comes on outside, and you are safe. I realized later that the button with a pictograph obviously representing someone talking would probably have repeated the most recent message for me, but at the time all I could think of was that I hoped it wouldn't talk to me any more. I hear that some play music.)
Here I must say a word about French toilets. All the ones I saw were designed to be low-flush, and hence were more cylindrical than the ones in the US, with smaller tanks. You almost always have a choice of two buttons to push to flush, depending on whether you want a big flush or a little flush. Either way, the tank seems to fill up quite quickly afterward, leaving me feeling, back here in the States, like my toilet is a huge water hog because the tank takes forever to refill. (I never saw a normal-flow toilet anywhere in Europe, which made me wonder why the low-flush technology hasn't caught on more in the southwestern US. I really did like the low-flush toilets, and I'd like to install one in my house.)
So in my first sanisette, I pushed the small flush button, and nothing happened except that a voice told me something useful. I was fairly rattled by this point; I got that it was telling me I had chosen an option that conserves water, but I couldn't figure out the rest. Maybe it's telling me the little flush is out of service? I pushed the big flush button, and got another useful response in French that went over my head, and then pushed the small button again once or twice, waiting for something to happen. Eventually I gave up, washed my hands, and departed, befuddled. I believe now that nothing happens until you push the button to exit, and then the bathroom goes into flushing/cleaning mode in preparation for the next person. I hope it took my multiple button pushes in stride.
One last word on this indelicate but essential subject. I had read online that toilet paper is often scarce in public restrooms in Paris, so I brought along about a dozen of those small packets of Kleenex that you can carry in a purse or pocket. For what it's worth, I never needed to use any of them. Even if I had needed some, I probably wouldn't have needed a dozen packets, so this was definitely overkill. At least I'm all set for the allergy season.