I've had the student and the teacher version of the card. The Prado lets you in free on both. We got into the Palacio Real for free with the student version. I found that of all the Western European countries, Spain provided the most "discounts" using that card. (And they say France is more of a patrimony to the arts...
)
Mostly the larger museums gave discounts. Absolutely no discounts on cathedral and other religious site visits, though.
Oh, one more thing: In 1993 I got double insurance payback for all of my medical bills through their travel insurance...I tore the ligaments in my left ankle, and after insurance payouts, actually MADE $1500!