Very good.
I am sure the book is better, but this is nevertheless a well made thriller, with nice international implications, almost 'Ludlum-like'.
If you do not notice that most of the movie was filmed in Croatia (then Yugoslavia) and that most of the 'Greeks' are actually Yugoslav actors (with their distinctly Slavic accents when speaking English), you will find this to be a rather involving movie, one that provides for some very decent entertainment.
For 'connoisseurs' of Greece, this looks very much like the Onassis - Livanos story of the 70's, but then again, this may just be my imagination...
The acting is very good throughout, especially the always egregious Omar Sharif (who basically plays himself) and also Jayne Seymour is also quite good (apart from being gorgeous as usual).
They don't make them like this any more.
Highly recommended.