Some thoughts after just finishing all the licenses and missions.
No words needed from me on the visuals - as others have said, they are stunning. But to be honest, all GT games have looked good enough to me at the time, it's the way the cars feel to drive that's most important.
I haven't played GT in over 18 months, but it immediately felt 'familiar' - if you've played GT before, it will feel very natural straight away. The cars have that 'always sliding a bit' feel once you get to the limit. Did most of the licenses with the pad, but got the wheel out for the missions.
I've been playing AC over the past year, and although it filled a gap, I never really clicked with it. The cars feel dead... no feeling of rotation in to the corners, even with mid engined cars, or when trail braking. I never thought weight transfer was modeled correctly in AC. And the tyre model has a very clearly defined grip limit - drive up to this and it works, drive beyond it, and the tyres heat up and lose grip far too quickly. I know some think it's the best thing since sliced bread, but for me, I hardly ever felt the desire to boot up the PS4 and just drive random cars... almost all my time was spent in the 991GT3 Cup.
As soon as I plugged the wheel in, GTS felt better.
Spent most time in the 991RSR (3 full runs on Mission 6-8)... it's glorious to drive with a wheel! Really wants to rotrate in to the corners, and really responds to trail braking - if the RSR is any guide, left foot braking will be a massive benefit in GTS... if you're carrying a touch too much speed mid corner, and the front is running a bit wide, you can dab the brakes and the front immediately pulls back on line... lovely. Other than that, the weight transfer just feels right - once settled, you can adjust the line minutely with the throttle... pushing a bit wide afer the apex, just fractionally ease the throttle and the nose comes back on line... as it should.
Plenty of weight in the steering (T300), much smaller deadzone than AC (virtually nothing), and although maybe the FF doesn't transmit surface detail as well as AC (eg; kerbs), it's vastly better at letting you know what the car is doing (over/under steer).
Also really enjoyed the 2 'autotest' missions, and the GTR one. You can keep the Vision cars though - they have no place in a GT game.
AI is mainly a massive step forward vs previous GT's... they don't just form a snake and follow the leader. And they have decent pace! They do occasionally punt you, but much less than previously.
Complaints? The rubber banding is just ridiculous in mission 6-8... twice I finished my last stop, exited with a 5-6s lead, only to find the M6 pull that back in a couple of laps despite me running within 0.5s of my best lap... and I closed a >20s lead on him earlier in the race in just 3 laps!
My only reservation about the full game is the nature of GT seems to be changing - there are very few ordinary road cars in the game, and these are primarily the most enjoyable for me. I won't drive race cars much, and then the Vision cars are a complete waste of time. Where are the classic GT cars like the NSX-R from the 90's?
By the way, once I finished the licenses and missions and collected my prize car, my demo locked and is now unplayable!
Edit - demo now playable again.