I've read a lot about getting student discounts on activities in Europe but I've been wondering if the ISIC card is really necessary for this. If I have a student ID card from my country or just state that I'm a student will that work? Similarly, are HI cards worth it or are there plenty of non HI hostels available at similar prices?
In smaller places there are often only HI hostels, but most major cities have a selection. Several people here feel more comfortable with HI hostels in general, and sometimes they are clearly the best choice, but in at least a few cases, they are not. So the bottom line is that you should consider your itinerary and get a rough estimate of how much time you will be spending at HI hostels. Then you'll know if the discount for getting the membership is worth it.
For ISIC, I would not expect anything other than that card to work. If you are counting on the discounts, get the card.