My thoughts so far after one playthrough:
Graphics -> A W E S O M E ...
Open World -> You only have a small area of the city in the demo but from what I've seen this is not only the most beautiful, but also the most alive open world I've ever seen in a game ... however we'll see how interactive it is. Original era playboy magazines are awesome and fit really well into the scene but there's too much useless interaction like flushing the toilets etc. which kinda gives the feeling they just added that to give the impression of big interaction possibilities.
Driving physics -> Nice, cars feel heavy
Up to that point I thought that this game has very good GOTY chances. Then came the mission however.
While I could imagine that the characters may grow to you after some time and that it's actually a major strongpoint of the game, I didn't really give a **** about those guys but then again it was a short demo so that was to be expected. But I didn't like the overall design of the mission, too much shooting imo. I hope that doesn't go through most of the missions because while the cover system works great and weapons feel ok(but should feel much stronger still), it would become boring very soon.(like GTA4s missions to some extent).
So in conclusion so far I have no doubt that this will be a very good game, but whether it's going to be a fantastic / GOTY-chance game will depend on how the other missions are structured and how much substance there is to the characters.