I'm about 50% through the singeplayer and I have to say, whilst this game looks great, plays great and has a decent enough story campaign there's just something about GTA games that means I begin to struggle to continue once it gets to about half way through. The same happened with GTA IV. I never actually finished it.
I've dabbled in the multiplayer though and have to say, I'm looking forward to getting my friends together for the heists. Tried a few with some randoms and was already good fun.