Since Al was to become Michael's Luca, I always felt that his hasty introduction (or non-introduction, really) in the movie was a shame. I, too, think that his sub-plot was very interesting in the book. I thought it was an excellent example of how Vito took Michael under his wing, was showing him the ropes. I also thought that Michael's handling of Al, inviting him to stay for dinner, sending him on the nice vacation to Florida, also showed a side of Michael that we really didn't see in the movie - just how he got these people to be loyal to him.

The one thing I found interesting about Neri's father-in-law is that he goes to the Corleones for help out of guilt, because his daughter had left Al because she was afraid of him. Al sounds like such a force in the book, and I don't think the character was ever developed fully in the movie.