Challenging? It's more like, "if you spend every moment of your life playing games and have the reaction time of a rabbit on some sort of mad crack, buy this game, it's a challenge". If you are a casual gamer go ahead buy the game but don't expect to get past the first level.
It was on my buy list once and then I rented it first (phew) and it was just too difficult and not entertaining.
However, my recomendations would be Forza (obviously) , Midnight Club 3 and DOOM 3.