I played the new Gran Turismo on the PS3 and it's really challenging. The one thing I noticed right off the bat is the weight transfer and more difficulty turning in. Under braking the nose of the car dips a LOT more compared to previous GT's, which makes it harder to brake (you have to start letting off the gas earlier and braking slower sooner before pressing hard on the brake or the car looses control and fish-tails). I give it two thumbs up because it's more realisitic than the previous versions. When you've been on a race track (literally, not just in a game) you quickly realize how un-realistic the games typically are. In my opinion the new Gran Turismo is pretty darned realistic, which is one of the reasons why it's harder to play.
PS: Sorry, I thought this post was about the new GT game, not GTA, haha.