Buying rF2 ruined every other 'sim' for me. Like i always say: When I race in rf2 with a good mod I forget that I'm behind my PC and get the feeling i'm racing a real car after a while. In every other sim I just feel like i'm playing a game.And it's ever evolving. The new tyre model feels so awesome, even though I don't like karts.
And there's still a lot more to come. If you like endurance racing it's a no-brainer since it simulates so many factors. No, it's a no-brainer for everyone who considers himelf a simracer.