Rewarding, but very difficult

Sciences-Po is a great institution and well-known in France. It is definitely a program for serious students who are interested in taking rigorous classes. The most challenging aspect of the program is simply the difference in educational culture. In France, the courses are much less structured, and reading lists can be very broad and listed as suggestions, rather than certain books that are required. (i.e - here are 30 important books, read 5-15 of them). Sciences-Po also has an essay style that is unique to the institution, where you list two arguments in favor of an argument, and then two against. That takes some adjustment for American students.
Overall, the experience was very valuable and the quality of the courses is very high, but you need to put in a lot of work and even very conscientious students are way outside their comfort zone on a regular basis.