Generally good , but the service nationally is suffering from government cutbacks at present .
In the smaller islands the little medical centres are usually staffed with younger graduate doctors fresh out of medical school , so their training and technology skills are up to date .
Also all are now linked with interactive computer and video so a specialist in one of the big hospitals in Athens can guide what a doctor is doing in the remote places .
Also nearly all the small islands have heliports now and the helicopter ambulances carrying medical staff can collect patients needing specialised treatment in emergencies and get them into the big Athens hospitals within an hour or two .
If you go there as a visitor then take any medicines you are currently prescribed with you ( to ensure you have the stuff available and to avoid putting any further burdens on a stretched service ) .
Take copies of any prescriptions , and a doctor's / hospital letter detailing your condition and the medication you have been prescribed , to prove the stuff you have with you is for you and not for resale .
Make sure you have good travel insurance including Medical repatriation .
Used the medical service on a couple of occasions over the years whilst on holidays and had excellent treatment.
My experience with them is good, Last June I was stung by a sea urchin, I went to a little hospital/doctors place which was a little run down I admit, and the man didn't speak English very well so it was hard to communicate with him however he did treat it and gave me a prescription.
they are good, better than in the US