It's been a while since I read them, and the plots are a little fuzzy now, but as I recall my main objection to the two books was that they relegated Corleone Family characters to fairly minor roles while going on in great detail about the adventures of Nick Geraci (Winegardner's alter ego) and other new characters we had never heard of.

And as I recall there were several whopping continuity problems where certain plot points in the books contradicted events which took place in the movies (especially GF III), which I consider ironclad canon.