The isic card is trully worth while. In Portugal you can get discounts in the Museums, Cathedrals and any type of Nacional Monument or Private that you may wish to visite.
As for trains there is also discounts. I am not aware of discounts for bus or metro, unless if you buy the monthly card in which case the isic card would give you a significant discount. I would like to warn you to buy the bus ticket on some shop and not in the bus. Ask the persons around, where to buy. In the bus it will cost you a little over the double price.

If you are a student and don’t get an isic card, bring still your student card, specially if it is easy to see that you are student (Date, University, Name and picture if possible). I believe that if you show that at any of the possible discounts they will still give you the discount. Remember that Portuguese people are very friendly so it is always worth while to try and speak a little bit.

Hope you have great fun in Portugal. I have lived in several countries, and i must say that Portugal has some certain magic around it… Hope you feel it!