Paris Sud is a great university from an academic point of view, the professors are highly qualified and the courses are well prepared to give you a solid background in the domain you are studying.
Studies are intensive and also the exams, you might have 2 exams per day for the midterms.
Some professors have to improve their English.
Buildings and facilities are a bit old.
Orsay Ville is far from Paris City and there no much facilities close by.