Sure there's plenty there to keep you occupied, but after the amount of time and money that went into I should be saying this is the best game ever made and I will still be playing it when GT6 comes out.
I loved GT4, but after about a year all I did was do time trials at the Nordschleife because the rest of the game had worn thin.
Sadly, its only been 2 months with GT5 and I'm already at that point. Had they developed a descent AI system, the game's lifespan would have been much longer.
Funnily enough, I really enjoyed racing the AI in Prologue. Not sure what they changed for the final game but its not good...