Hi MsRobinson,
I did all my higher education at Sciences Po, and am just finishing the Master of Economic Law before starting my PhD at Cambridge. I am also close to the research community at the Law School; I think I can give you a few elements on this Master.
A few words of general context first: although Sciences Po is a venerable school, the Law School is very young: not even a decade old. It was created by professors from the university disappointed with the traditional form of legal education there (you can't, under French law, teach in English in the universities, for example). And so, as opposed to the code-article-by-code-article style of teaching you would find in Law Faculties, Sciences Po's point of focus is Legal Realism on steroids.
Sciences Po Law School is a great place to study; the courses are for the most part interesting, teachers speak English well, and the academics are well connected with their peers in the best international universities. PhD Students there love it: they say they are constantly intellectually challenged in the course of their PhDs, and the connections with other scholars are great. The student body is very diverse, and, come on, the Boulevard St Germain area is incredibly beautiful. It is a small school, a small faculty, and this is great: you can know everyone around you, which helps in building the personal ties and relationships that lead to the best future opportunities. In short, the quality of the whole master, on French standards, is great.
But here is the issue: this is on French standards, which are not very high. Sciences Po Law School is great for you if you want to enter the French legal market (and then, only in the top, foreign-originated law firms, for the old French law firms believe it is not as good as traditional Law Faculties are...), but a lot of degrees from UK/US/Swiss law schools would be as helpful for that. I have also studied at Columbia Law School, for instance, and the difference in terms of ressources, name recognition, etc. are staggering. Sciences Po is fairly recognized abroad and in Europe, but again you would perhaps struggle more than with another equivalent UK/US degree.
Similarly, if you intend to become a professor, a degree from Sciences Po, even a PhD, wouldn't take you very far - not only abroad, for Sciences Po is not that well recognized, but even in France, for traditional Law Faculties hate us.
So there are pro and cons, depending on what you intend to do. And, as usual, a lot depends on what you do once there: there are plenty of opportunities to benefit fully from this Master, if you want it, but it requires to personally establish the links with the people, the community, etc., which would help you further up the ladder.
Hope it helped,
Youpla