The point is that you can "finish" the game in 2 weeks with the exception of some of the endurance races. The next 6 months (to collect all the cars and finish endurance) are spent repeating the same races over and over again.
I have done 115,000 miles and have about 1040 cars with only a few duplicates. I have worked out that I need approximately 130,000,000 credits to get the rest of the cars.
Even if I do the most lucrative race - The American Indy race I only get 98,000 credits that mean I need to do it 1326 times and that would take 110 hours. At 3 hours a day it would take me 37 days of doing the same race over and over. Is that your idea of a fun game ?
As I am fed up with grinding I now run B-Spec Dream Car Championship for 750,000 credits per hour.
Whilst that is happening my PS3 cannot be used for anything else so I bought an Xbox360 at weekend and Forza 3 to give me something to do. I swore I would never buy an Xbox before GT5 came along.
Sony's flagship game has actually made this lifelong Playstation man buy an Xbox360 !!