If you've asked yourself: "Why should I get this game when I already have Project Cars / IRacing / Gran Turismo 5-6 / Forza?"
This is why you should get the game.
But to be serious, this game has a very solid base from which people can modify the game with. The physics are top notch and I firmly believe that Assetto Corsa is second only to Live For Speed in the physics department. Every car you drive feels very different from each other and have realistic differences between them.
Yes IRacing has better multiplayer and a very in-depth league / community system.
Yes Project Cars looks and sounds much better.
Yes, Forza and Gran Turismo have a very large list of cars and things to do.
But if what you desire is a challenging yet very accurate sim racer, this is pretty much it. Every car handles beautifully and the ones with excellent electronic assists (McLaren MP4-12C or Ferrari LaFerrari for example) are insanely fast with them and difficult to master without them.
You hear car reviewers say that the LaFerrari is very well balanced and easy to control and you get that in Assetto Corsa. The car is actually easy to control! It doesn't fight you, it doesn't whip around the corner and spin, and it turns in sooo nicely. The issue is that the car accelerates so fast you're hurtling towards the corner at unbelievable speeds and miss the braking points entirely. At that moment, you're reminded that while the LaFerrari has more horsepower than a modern F1 car, it doesn't have the aerodynamics, brakes, weight, or tires of one and then you slam nose first into the wall for overestimating it's braking power.
This game is great fun for those who enjoy driving at the limit and feeling every cars' characteristics since it emulates that the best. Likewise, you'll get the most enjoyment if you had a racing wheel, but a controller is fine.