GT5 is fun for me as well, but GT-PSP aside, it is the least amount of fun I've ever had with a Gran Turismo title.
I am an A-Spec man, I like to play offline, tune my cars and use them in the racing series the A-Spec mode offers me. But GT5 is not very good in any of that:
- there are not enough race series, often not enough races in the series, the track selections are pretty bad, and the financial result is much less than satisfying. I get prize cars only once, and the amounts of credits are to small
- car tuning options are somewhat limited, you can hardly spoil a car by bad tuning anymore, only a handful of cars can be race modded
- playing against the AI is no fun either, because it is only half done. In earlier GTs the AI was at least predictable, so I was able to set up my car that I had to put up a fight to win. When I do that in GT5, they take me out in the bends or brake in the middle of the straights
So what do I end up with? A game that could have been significantly better without any more effort by the producer. Had they just taken Prologue, added cars and tracks and an optically souped-up GT mode from GT4, this would have been a much better game for me. All the great things they obtained: WRC, Nascar, Top Gear and whatnot - all wasted in mediocre challenges which either are much too easy, too hard or simply make no sense at all.
When I am in cockpit view, racing my individually-tuned car around a history-charged track, GT5 is great! But as soon as I lift my head above the surface of this little universe, all those little foibles and and inconsistencies start hitting my face. That makes me think twice about doing that again, but playing through GT5 I hardly have a choice.