Depends where you're going and what you want to do. If you plan to see most of the main sights, museums etc in western Europe, it can save you $$$, but if you're travelling in eastern Europe, Asia etc. it won't be as useful. A lot of places we went wouldn't accept it, even some that had the sign on the window! Usually the travel guides list the ISIC prices for various places, so if you figure out generally which ones you seriously plan to see, tot up and see if it would pay for itself. If it's within a couple of dollars either way, I'd say it's worth it, because you may go to some other places or be able to use it at restaurants or bars as well.
Another little known tip, if you're a member of certain museums, zoos, aquaria, or other cultural associations already, sometimes they have reciprocal privileges with others around the country and around the world. So if you already are a member, it's worth checking with their membership office to see if that can save you the price of admission on your journey.