I just played this game for six hours and had to pull myself away for the sake of my eyes. My god, this game is incredible. I never owned any of the PS2 GTAs, only played them at friends' houses. I loved GTA IV, and already this game kicks the shit out of it.
The world is so lovingly crafted and dynamic. The lighting is beautiful, the random encounters are awesome, I can just drive around Los Santos forever. I think my favorite detail is how appropriate the NPCs are. Hipsters hang out in front of the Urban Outfitters, backpackers stand atop Mount Chilliad and take photos with their phones, rednecks drink beer behind liquor stores on the outskirts of town. It just makes the world feel so much more lived in and real.
And the world is just filled with fun things to do. I'm not talking about mini-games either, but things like barreling down a mountain trail on a dirtbike or hunting.
I'm going to be playing this game for a long time.