I don't actually own Forza 5...or even an Xbox One for that matter, but I did get a couple of races in with a friend of mine @RagonKai and I thought I would share my first impressions.
About the Game:
It's a racing game...not much else to say, other than I suppose I should point out that it is geared more toward the simulator fans rather than anyone in the arcade scene.
What I liked:
Graphics don't make a game, but "Next Gen" shininess is certainly the first thing you notice. Other than that I think all my other positive feelings during the races were more to do with the new console rather than the game itself..
What I didn't like:
It seemed fairly limited..not many cars, even fewer tracks. This could have been put down to RagonKai's current progression in the main game but I wouldn't know about that so I can't really say.
I mentioned graphics as something that I liked, but to be honest, I expected more. As a PC gamer I am fairly critical of 3D models and textures, there are a lot of shiny effects and I can't fault the lighting but the design itself felt a little flat (especially scenery/buildings).
My final criticism is about the actual "simulation" element of the driving experience. When you are traveling at 90mph, the car will corner like you are doing that sort of speed..but to look at it in 3rd person view..it just feels like snail's pace. I found the best way to alleviate this sensation of slowness is to go to the bumper cam.
Forza 5 is like Forza 4...which in turn was like Forza 3 through 1. The game doesn't really change, save for a few tweaks (not necessarily good ones)
It is what it is and no more, not much else to say on the subject!