(NOT interested in a flame war, this is just a throwaway post to get my first 5 behind me)
Never even thought about buying a Playstation until the original Gran Turismo came out. Went on to buy a PS2 and pretty much only played GT2, GT3, and GT4 on it. Bought an original 60GB PS3 in anticipation of GT5 and waited...bought GT5 Prologue and waited...
GT5 finally ships and WTF? Horns, driving suits, helmets, and gloves, while only a subset of the cars are "Premium" cars? Found myself playing Forza 3 (and now 4) much more. Every car is a "premium" car and I don't need a friggin' color chip to paint my cars...oh, and the Game Shark is built-in
Digging deep, about the only thing I can find redeeming about GT5 is I believe its simulation engine is still a bit better than F4, but not by much. You can feel the effects of tuning in both games and both are consistent enough to allow you to continually improve your lap times.