I have played Gran Turismo 5. It takes like an hour to install to the system. The first level I played was a snow level with almost all of the pussy options disabled (ABS, added traction, lane-alignment, etc) (I think I had overall traction set to 3/10, can't recall) and oh god if you're trailing someone you can't see shit, even with the wipers on full blast. It was a riot.
The biggest pitfall for the "realistic driving simulator" that I've witnessed is that you're not penalized for slamming into your opponents. Because they, seasoned driving veterans, always take turns smoothly, I, the up-and-comer-who-knows-how-to-drive-well-but-knows-how-software-works-even-better, realize that if I just pop it into fourth gear, slam on the gas, neglect the bend, and just slam into my opponents, I get re-directed on the track without any negative consequences, and get put ahead. Every cluttered turn, I'd gain 3 positions, because I just didn't need to turn properly until I was in first place.
Still, entertaining, but I'm not such a connoisseur of racing simulation games that I can tell any difference between Gran Turismo 3 or 5 or Forza 3 or what-have-you. They're all the same to me.