Hi,
My student ID looks like a toddler played with photoshop. Everybody with a laserprinter could fake the ID. However, while traveling in Europe I was just sometimes asked to show another ID, because my student ID doesn't have a photograph. I've never owned a ISIC.
As long as you look young and have something you are mostly fine.
The amount of student discounts differ quite a lot. Mostly you get only discounts in at museums, cinemas and something like this. For public transport a student id is in Germany often worthless, since the Universities and the public transport provider have special agreements, which are only open to enrolled students.
cheers
757DUD

PS: For most of the student discounts in Greece, you are required to be a citizen of the European Union.