I am a little bit of the way through Fools Die and I must say I am enjoying it, although it's true it has it's good bits and slow bits.
The Dark Arena was a book I enjoyed. That is all I have read so far; The Godfather, The Dark Arena and some of Fool's Die.
I do know that Puzo himself considered The Fortunate Pilgrim to be his favourite book.

Winegardner? Trash. I made it to page 22, upon which I found myself to be going in a bad mood. I haven't picked it up since, and I'm fairly positive I could happily live my life not knowing what happened in "The Lost Years" if it's gonna come from his bolshy, unnatractive writing.