I played the demo last night and here is what I experienced.
First a brief summary of my PC specs:
i7 7700k
GTX1080ti
16 gigs DDR4 RAM
So immediately I turned all of the settings up to ultra. I turned off all of the dynamic settings and cranked it to the max. First I played the game in 1080p on my PC monitor because I won't invest in a 4k until I can get one with at least 144hz at a reasonable price. So basically in like 3 years. Anyway, the game held a rock solid 60fps and looked gorgeous. Definitely the best looking Forza to date. Then I tried it out on my 4K TV. I didn't think 4K would make much of a difference, but wow did it look great. The detail is incredible. Again, the game ran at a rock solid 60fps with everything cranked all the way up. I was impressed.
One thing I wasn't impressed about in terms of graphics is the game is capped at 60fps on PC. This makes no sense to me and I'm hoping it will be patched at a later date. I can understand the cap on console, but not PC. Also navigating the menus was an annoying experience. They were choppy and just not smooth most of the time. It was really strange and I'm hoping it's just a bug in the demo. What I found odd was the choppiness was not as frequent when running the game at 4K. This was a minor annoyance though.
In terms of gameplay, it's about what you would expect from a Forza game. It's not a sim or an arcade, but somewhere in between. I believe the new term thrown around the internet is "simcade" racer. Anyway, if you love the way Forza feels, well, it's still Forza. Now I only played the game with an Xbox One controller. I wish I had a wheel to test, but I don't. On that note, I've heard very bad things about wheel compatibility and a lack of force feedback, which is a huge disappointment.
Even though I enjoyed the gameplay, the Porsche 911 GT2 RS didn't feel right. It just didn't feel like a Porsche. There was a lot of understeer and it was really easy to lose the back end with even a slight amount of throttle input in the corners. I'm sure this can be fixed with tuning, but a stock GT2 RS would handle better than it does in the game. To give an example, think of a 900HP car on a road course with worn out street tires. That's basically how it felt.
Anyway, I've got the game preordered. I am a big Forza fan, so I'm pretty loyal to the series. Regardless of my loyalty, I think the game would be a solid buy for anyone who isn't looking for a hardcore sim, but also doesn't want something too arcady like something out of the Need for Speed series.
Just be weary of the wheel support. On the Forza forums, it's pretty much universal that there are no wheels out there that feel right in the game. If you were looking forward to the game and have been dying to play it with your fancy wheel, you might want to wait to pick it up or avoid it all together if this is a deal breaker.