It's one of the best games I have ever played (and I've played a lot), but it honestly could be much better.
It's the Mafia theme that sets GTA and Godfather apart. You can extort businesses (apply "pressure" if they don't respect you enough; a revolutionary system), buy out rackets, and participate in mob wars. Anyone who's only interested in a 1940's GTA experience can get that, too.
Much detail has been put in to ensure that this is Godfather to the bone. The character details are amazingly comparable to the original actors, and the voicework is the same, too (most of the old actors returned to create a classic experience). The only thing that bothers me about this is that Al Pacino's character is missing (he wasn't interested), and it just isn't the same without him.
On graphical terms, this looks really good, better than GTA. You can feel the atmosphere of the 1940's all around you. However, all of the interiors are repeated to a laughable amount; as I slowly took over New York, I especially noticed this.
Despite some of these problems, Godfather is a refreshing change of pace. Those who are fans of the movies should do themselves a favor and pick this up.