Been playing for 45 minutes so far.
Rudimentary performance report with an i7-4790K and a GTX 980@1080p with latest drivers:
I put all of my settings on *almost* max; save for AA (4x), Reflection AA (2x), and motion blur (don't want too much of that stuff). There's also a separate menu for "advanced graphics" with some kind of enthusiast settings that I decided not to touch.
Performance is good; with Vsync, I'm able to maintain 60FPS maybe 75% of the time, with lots of dips. As a quick fix, I bumped down AA from 4x to 2x, and disabled Nvidia TXAA. Now I'm getting almost a constant buttery-smooth 60fps. (A Nvidia-exclusive feature tanking performance, who would've thunk it? )
I'm also playing exclusively in first-person camera view. It will feel extremely cramped at first, but fortunately there's a FOV slider. No numerical value is shown underneath the slider, so it's a little vague; but from my rough measurements, max FOV appears to be a perfect 90-degree angle, which is where you should definitely put it. Feels great after that.
I also gotta say, damn, just walking outside of Franklin's house for the first time, and looking around... this is one of the most detailed games I've ever played. Like, ever. Not just in the graphics; but rather, in the amount of effort they put into making a living, breathing city. It feels so real.