Think very hard final exams at the end, long history behind the program and prestige associated to that. It's not nearly as hard to get in as to excel.
I think the attraction is that it's one year, and you get to spend it brutally working through what they offer, and they tend to teach you more than you would generally absorb in that period of time.
If you want a more on-your-own-time researchy experience, you should look to do something else. Some top students love doing this program, and some would avoid it, no-brainer.