Playing in 4K with HDR. Hands down the best looking game I've seen - it is, for the most part, photo realistic. There's some funky shadows, some LOD issues here and there, and some of the environment assets (mostly trackside) can look a bit tacky - but as an overall image it is stunning. The gifs/replays I saw earlier had my expectations high and I was skeptical that gameplay would look the same - but they've done an incredible job.
It's been a long time since I've played a Gran Turismo game. I played GT2 heaps, never touched GT3, GT4 or prologue/PSP. I bought GT5 on PS3 but took it back almost immediately because the visuals and audio were all over the place and I was enjoying arcade racers at the time. So I was unsure what I'd think of the physics after so many years of Forza, Shift and arcade racers. Again, they've nailed it. I have no idea how the physics actually stack up, but cars certainly feel alot more 'planted' to the road than other games. As a result, the tracks feel great and the act of shifting gears, braking etc - are all really good, as you can feel the car shifting weight.
Most importantly, I think they've made a great choice by focusing on online. There's plenty of content for eveyone, but the DR/SR system will be great for close, fair races - something that racing games (at least on console) have been lacking for some time.
That said, the game isn't perfect. I think the audio is very good overall, but some of the cars (WRX STI for example) are lacking. I also think the tire squeal can be excessive - hopefully they give us the option to adjust the sound mix.
I'm not sure on car variety. There are plenty of cars, but going to the Brand Showrooms can be a little discouraging. Also, does anyone know what the benefit of owning cars in your garage is? I appears that most races let me select a wide selection of cars that I am free to modify - so I don't know why I'd buy a car? Guessing there's a reason but it's just not clear for me yet.
Finally, where is the modification? I knew it wasn't going to be Midnight Club with body kits etc. However I always loved taking cars and spending tons on upgrades, and then tuning and testing. I'm okay without it, but some limited options outside of livery would be good.