I’ve used this card a lot whenever I have traveled in the past and decided to get one before heading to China for a year. However, once I got there I don't think I used it at all. I read in Lonely Planet that a few hostels in bigger cities took it, but budget hostels are pretty easy to find, so it was never an issue. It never hurts to carry a student ID with you, and if you already have one from University then it will probably work fine. Odds are that you probably will not use the ISIC, but... it’s only like 20 bucks in the states, so it won’t set you back too much.
I do remember using my student ID from home (not the ISIC) for places like Yellow Mountain, the Great Wall, and the Forbidden City.. so bring a student ID, but it doesn't need to be the ISIC.