There was a LOT more culture shock than I was expecting, but by the end of the program I truly felt that I had gotten everything out of it that I had wanted coming in. My French proficiency grew by leaps and bounds, I felt much more confident as a person and like I had grown and matured significantly, and I had some amazing travel experiences. Also became friends with a great group of girls (and one lone guy).
The volunteer position was a surprise at first- when my group signed up, all of the information was still saying that it was optional, when it had in fact been made mandatory. I understand that some people liked their placings less than others, but I found mine (Belleville) to be very chill (for Paris) and not a bad way to spend a few hours a week. It was probably the area I felt least safe in, especially in the winter months when I was generally traversing it in the dark. I generally am not comfortable around kids, but I had enough support from the regular volunteers and staff that I had the authority to keep them in line. And trying to teach subtraction in French is an excellent bit of baptism by fire for improving your language skills.
Housing was fine, nothing horrible, nothing special. At my own request, I did not have a roommate, but the only Single offered by the program put me out in the suburbs when I would have preferred to have been closer into the main action. Still, it was in a safe, cute neighborhood, and very convenient to Catho. Food was provided (I didn't partake, as it wasn't very vegetarian friendly).
The program director, Shelley, was an absolute godsend. There were many times when we all thought she was totally misleading us about something, and then it would turn out that she was completely right. Literally every single time. She gave us our space, but each time I reached out about something, she went above and beyond to help me out. In my case, she was particularly useful with finding vegetarian options (she located a list of every vegetarian and vegetarian-friendly restaurant in the entire city of Paris, and gave me tips about ethnic grocers where I could find supplies of tofu and the like), and getting rid of creeps on the Metro.
The creeps on the Metro were one of Paris' biggest failings in my book. Also the fact that n.o.t.h.i.n.g is open past 9pm, or at all on Sundays. Learn some choice phrases to use on the Metro first thing, or do as I did, and take the bus! It might take a little longer, but it's mainly populated by children and old people, and you can actually see Paris while you commute!
I suppose we should actually get to classes at some point. Classes were startlingly long- 2 hours at Etoile and then 3 hours and Catho, with 10 minute breaks at the halfway point. Also, entirely in French. However, I found that they were real confidence boosters. Assuming you didn't cheat on your placement exams somehow, the professor spoke to your level of French, so as long as you stayed alert, you could readily understand all three hours. The homework load was quite reasonable, and the professors relatively friendly.
At school, we met a lot of other American students with different programs. They lived with other American students, and often only took one class at Catho, and the rest were taught by their program, in English. Generally speaking, those of us with Central were speaking with a greater level of ease by the end of the semester. The program Shelley has put together for Central is demanding. We have to use French in our day-to-day life more than English, negotiate with French people more than Americans, and get out into different parts of Paris frequently. If you want a complete fluff semester, this is not the one for you. You will be challenged, but you will grow. It is worth it.
A final note: Paris has weather almost identical to Seattle (lots of grey in the winter and spring, gorgeous in the summer). Be prepared mentally.