I just started reading The Sicilian last week, but I keep getting interrupted with school work, they've got me reading The Importance of Being Ernest so I had to stop with The Sicilian. But what I've read of The Sicilian so far, I'm about half way, I really like.
As for Mario Puzo's other novels, I love all his books and ones like The Fortunate Pilgrim and Fools Die are just as good as the Godfather, in my opinion.