Is it worth investing in Greek real estate property ?

Absolutely yes . 

Athens is an emerging market and  popping right now;  you can buy apartments in the center of  Athens for $15K-35K, renovate them for $5K , furnish them and  generate $1K per month (through Airbnb, and other platforms which have the similar usage ). 

Apartments in Athens are very cheap (you can buy apartments for up to 15k). 

Athens receives too much tourism traffic and it is loaded for 8 or 9 months of the year. 

This gives the opportunity to the landloards to rent in in Airbnb and to have a very serious income .