Jesus. I probably played ten hours today. I haven't played a game like this since Skyrim.
Was the same for me. And a ton of it wasn't even straight gameplay. It was me fucking around with shit like tennis, golf, sidestuff, etc.
The game is a great validation of what a big budget game can be when you spend a shitload of money and resources and time. Its like the anti-assassin's creed 3. I actually get a little sad when I save and the completion percentage is higher than it was before.
And just to pull a completely random comparison. The original Bioshock came out a year before GTA 4. And I liked bioshock infinite. But it did not feel like a product that was that many years in the making. This does.
Also the first game in a while that just about everybody is playing on my friend's list.