Imo GT5 only gets fun after you really get going in the game. By that I mean you get a nice bit of credits and buy some nice fast, agile cars.
In GT3, GT4 and now GT5, I started out by grinding on the sunday cup, then grinding on the clubman cup, building up cash, then in the professorial league I actually start doing all of the races, although I also like to grind on the supercar festival at Daytona with my R8. Once I get to the expert and extreme leagues I then finish everything and go back and finish all of the easier races with overkill cars, and start collecting cars and doing some more serious online racing.
